--- a/dom/base/Navigator.cpp
+++ b/dom/base/Navigator.cpp
@@ -2742,28 +2742,16 @@ ToCString(const MediaKeySystemConfigurat
     str.AppendLiteral(", audioCapabilities=");
     str.Append(ToCString(aConfig.mAudioCapabilities.Value()));
   }
   if (aConfig.mVideoCapabilities.WasPassed()) {
     str.AppendLiteral(", videoCapabilities=");
     str.Append(ToCString(aConfig.mVideoCapabilities.Value()));
   }
 
-  if (!aConfig.mAudioType.IsEmpty()) {
-    str.AppendPrintf(", audioType='%s'",
-      NS_ConvertUTF16toUTF8(aConfig.mAudioType).get());
-  }
-  if (!aConfig.mInitDataType.IsEmpty()) {
-    str.AppendPrintf(", initDataType='%s'",
-      NS_ConvertUTF16toUTF8(aConfig.mInitDataType).get());
-  }
-  if (!aConfig.mVideoType.IsEmpty()) {
-    str.AppendPrintf(", videoType='%s'",
-      NS_ConvertUTF16toUTF8(aConfig.mVideoType).get());
-  }
   str.AppendLiteral("}");
 
   return str;
 }
 
 static nsCString
 RequestKeySystemAccessLogString(const nsAString& aKeySystem,
                                 const Sequence<MediaKeySystemConfiguration>& aConfigs)

--- a/dom/media/eme/MediaKeySystemAccess.cpp
+++ b/dom/media/eme/MediaKeySystemAccess.cpp
@@ -495,46 +495,16 @@ IsSupportedVideo(mozIGeckoMediaPluginSer
   }
   if (IsVP9ContentType(aVideoType) && aKeySystem.EqualsLiteral("org.w3.clearkey")) {
     return GMPDecryptsAndGeckoDecodesVP9(aGMPService, aKeySystem, aVideoType, aDiagnostics);
   }
   return false;
 }
 
 static bool
-IsSupported(mozIGeckoMediaPluginService* aGMPService,
-            const nsAString& aKeySystem,
-            const MediaKeySystemConfiguration& aConfig,
-            DecoderDoctorDiagnostics* aDiagnostics)
-{
-  if (aConfig.mInitDataType.IsEmpty() &&
-      aConfig.mAudioType.IsEmpty() &&
-      aConfig.mVideoType.IsEmpty()) {
-    // Not an old-style request.
-    return false;
-  }
-
-  // Backwards compatibility with legacy MediaKeySystemConfiguration method.
-  if (!aConfig.mInitDataType.IsEmpty() &&
-      !aConfig.mInitDataType.EqualsLiteral("cenc")) {
-    return false;
-  }
-  if (!aConfig.mAudioType.IsEmpty() &&
-      !IsSupportedAudio(aGMPService, aKeySystem, aConfig.mAudioType, aDiagnostics)) {
-    return false;
-  }
-  if (!aConfig.mVideoType.IsEmpty() &&
-      !IsSupportedVideo(aGMPService, aKeySystem, aConfig.mVideoType, aDiagnostics)) {
-    return false;
-  }
-
-  return true;
-}
-
-static bool
 IsSupportedInitDataType(const nsString& aCandidate, const nsAString& aKeySystem)
 {
   // All supported keySystems can handle "cenc" initDataType.
   // ClearKey also supports "keyids" and "webm" initDataTypes.
   return aCandidate.EqualsLiteral("cenc") ||
     ((aKeySystem.EqualsLiteral("org.w3.clearkey")
     || aKeySystem.EqualsLiteral("com.widevine.alpha")) &&
     (aCandidate.EqualsLiteral("keyids") || aCandidate.EqualsLiteral("webm")));
@@ -604,44 +574,16 @@ GetSupportedConfig(mozIGeckoMediaPluginS
   }
 #endif
 
   aOutConfig = config;
 
   return true;
 }
 
-// Backwards compatibility with legacy requestMediaKeySystemAccess with fields
-// from old MediaKeySystemOptions dictionary.
-/* static */
-bool
-MediaKeySystemAccess::IsSupported(const nsAString& aKeySystem,
-                                  const Sequence<MediaKeySystemConfiguration>& aConfigs,
-                                  DecoderDoctorDiagnostics* aDiagnostics)
-{
-  nsCOMPtr<mozIGeckoMediaPluginService> mps =
-    do_GetService("@mozilla.org/gecko-media-plugin-service;1");
-  if (NS_WARN_IF(!mps)) {
-    return false;
-  }
-
-  if (!HaveGMPFor(mps,
-                  NS_ConvertUTF16toUTF8(aKeySystem),
-                  NS_LITERAL_CSTRING(GMP_API_DECRYPTOR))) {
-    return false;
-  }
-
-  for (const MediaKeySystemConfiguration& config : aConfigs) {
-    if (mozilla::dom::IsSupported(mps, aKeySystem, config, aDiagnostics)) {
-      return true;
-    }
-  }
-  return false;
-}
-
 /* static */
 bool
 MediaKeySystemAccess::GetSupportedConfig(const nsAString& aKeySystem,
                                          const Sequence<MediaKeySystemConfiguration>& aConfigs,
                                          MediaKeySystemConfiguration& aOutConfig,
                                          DecoderDoctorDiagnostics* aDiagnostics)
 {
   nsCOMPtr<mozIGeckoMediaPluginService> mps =

--- a/dom/media/eme/MediaKeySystemAccessManager.cpp
+++ b/dom/media/eme/MediaKeySystemAccessManager.cpp
@@ -164,20 +164,17 @@ MediaKeySystemAccessManager::Request(Det
     aPromise->MaybeReject(NS_ERROR_DOM_INVALID_STATE_ERR,
                           NS_LITERAL_CSTRING("GetKeySystemAccess failed"));
     diagnostics.StoreMediaKeySystemAccess(mWindow->GetExtantDoc(),
                                           aKeySystem, false, __func__);
     return;
   }
 
   MediaKeySystemConfiguration config;
-  // TODO: Remove IsSupported() check here once we remove backwards
-  // compatibility with initial implementation...
-  if (MediaKeySystemAccess::GetSupportedConfig(keySystem, aConfigs, config, &diagnostics) ||
-      MediaKeySystemAccess::IsSupported(keySystem, aConfigs, &diagnostics)) {
+  if (MediaKeySystemAccess::GetSupportedConfig(keySystem, aConfigs, config, &diagnostics)) {
     RefPtr<MediaKeySystemAccess> access(
       new MediaKeySystemAccess(mWindow, keySystem, NS_ConvertUTF8toUTF16(cdmVersion), config));
     aPromise->MaybeResolve(access);
     diagnostics.StoreMediaKeySystemAccess(mWindow->GetExtantDoc(),
                                           aKeySystem, true, __func__);
     return;
   }
   // Not to inform user, because nothing to do if the corresponding keySystem

--- a/dom/media/test/test_eme_detach_media_keys.html
+++ b/dom/media/test/test_eme_detach_media_keys.html
@@ -14,17 +14,17 @@
 
 SimpleTest.waitForExplicitFinish();
 
 const keysystem = 'org.w3.clearkey';
 
 function createAndSet() {
   return new Promise(function(resolve, reject) {
     var m;
-    navigator.requestMediaKeySystemAccess(keysystem, [{initDataType: 'cenc'}])
+    navigator.requestMediaKeySystemAccess(keysystem, [{initDataTypes: ['cenc']}])
     .then(function (access) {
       return access.createMediaKeys();
     }).then(function (mediaKeys) {
       m = mediaKeys;
       return document.getElementById("v").setMediaKeys(mediaKeys);
     }).then(function() {
       resolve(m);
     });

--- a/dom/media/test/test_eme_initDataTypes.html
+++ b/dom/media/test/test_eme_initDataTypes.html
@@ -103,17 +103,17 @@ function PrepareInitData(initDataType, i
     return new TextEncoder().encode(initData);
   } else if (initDataType == "webm") {
     return StringToArrayBuffer(atob(initData));
   }
 }
 
 function Test(test) {
   return new Promise(function(resolve, reject) {
-    navigator.requestMediaKeySystemAccess('org.w3.clearkey', [{initDataTypes: ['keyids']}]).then(
+    navigator.requestMediaKeySystemAccess('org.w3.clearkey', [{initDataTypes: [test.initDataType]}]).then(
       (access) => access.createMediaKeys()
       ).then(
         (mediaKeys) => {
           var session = mediaKeys.createSession(test.sessionType);
           session.addEventListener("message", function(event) {
             is(event.messageType, "license-request", "'" + test.name + "' MediaKeyMessage type should be license-request.");
             var text = new TextDecoder().decode(event.message);
             is(text, test.expectedRequest, "'" + test.name + "' got expected response.");

--- a/dom/media/test/test_eme_requestKeySystemAccess.html
+++ b/dom/media/test/test_eme_requestKeySystemAccess.html
@@ -523,60 +523,16 @@ var tests = [
     options: [
       {
         initDataTypes: ['cenc'],
         videoCapabilities: [{contentType: 'video/mp4'}],
       }
     ],
     shouldPass: false
   },
-
-  // Test legacy support. Remove when we remove backwards compatibility.
-  {
-    name: 'Legacy CENC',
-    keySystem: CLEARKEY_ID,
-    options: [
-      {
-        initDataType: 'cenc',
-      }
-    ],
-    expectedConfig: {
-      label: ''
-    },
-    shouldPass: true,
-  },
-  {
-    name: 'Legacy CENC + MP4 video',
-    keySystem: CLEARKEY_ID,
-    options: [
-      {
-        initDataType: 'cenc',
-        videoType: 'video/mp4; codecs="avc1.42E01E"',
-      }
-    ],
-    expectedConfig: {
-      label: ''
-    },
-    shouldPass: true,
-  },
-  {
-    name: 'Legacy CENC + MP4 video + MP4 audio',
-    keySystem: CLEARKEY_ID,
-    options: [
-      {
-        initDataType: 'cenc',
-        videoType: 'video/mp4; codecs="avc1.42E01E"',
-        audioType: 'audio/mp4; codecs="mp4a.40.2"',
-      }
-    ],
-    expectedConfig: {
-      label: ''
-    },
-    shouldPass: true,
-  },
 ];
 
 function beginTest() {
   Promise.all(tests.map(Test)).then(function() { SimpleTest.finish(); });
 }
 
 if (!IsMacOSSnowLeopardOrEarlier()) {
   SimpleTest.waitForExplicitFinish();