diff --git a/android_webview/javatests/src/org/chromium/android_webview/test/KeySystemTest.java b/android_webview/javatests/src/org/chromium/android_webview/test/KeySystemTest.java
index daad6feffaf38..040a348676a05 100644
--- a/android_webview/javatests/src/org/chromium/android_webview/test/KeySystemTest.java
+++ b/android_webview/javatests/src/org/chromium/android_webview/test/KeySystemTest.java
@@ -139,14 +139,6 @@ public class KeySystemTest extends AwParameterizedTest {
     public void testSupportPlatformKeySystem() throws Throwable {
         Assert.assertEquals(
                 getPlatformKeySystemExpectations(),
-                isKeySystemSupported("x-com.oem.test-keysystem"));
-    }
-
-    @Test
-    @Feature({"AndroidWebView"})
-    @SmallTest
-    public void testSupportPlatformKeySystemNoPrefix() throws Throwable {
-        Assert.assertEquals(
-                "\"NotSupportedError\"", isKeySystemSupported("com.oem.test-keysystem"));
+                isKeySystemSupported("com.oem.test-keysystem"));
     }
 }
diff --git a/android_webview/test/shell/res/values/config.xml b/android_webview/test/shell/res/values/config.xml
index cf4e355851016..6fe6a42a5057e 100644
--- a/android_webview/test/shell/res/values/config.xml
+++ b/android_webview/test/shell/res/values/config.xml
@@ -9,8 +9,6 @@
     <string-array name="config_key_system_uuid_mapping" translatable="false">
         <!-- Use Widevine's UUID to pass the availability-test of MediaDRM plugin -->
         <!--suppress TypographyDashes -->
-        <item>"x-com.oem.test-keysystem,EDEF8BA9-79D6-4ACE-A3C8-27DCD51D21ED"</item>
-        <!--suppress TypographyDashes -->
         <item>"com.oem.test-keysystem,EDEF8BA9-79D6-4ACE-A3C8-27DCD51D21ED"</item>
     </string-array>
 </resources>
diff --git a/media/base/key_systems_impl.cc b/media/base/key_systems_impl.cc
index e7517a708a2a5..fb3f5743a8c35 100644
--- a/media/base/key_systems_impl.cc
+++ b/media/base/key_systems_impl.cc
@@ -24,7 +24,6 @@
 #include "media/base/mime_util.h"
 #include "media/cdm/clear_key_cdm_common.h"
 #include "media/media_buildflags.h"
-#include "third_party/widevine/cdm/widevine_cdm_common.h"
 
 namespace media {
 
@@ -187,56 +186,6 @@ class ClearKeyKeySystemInfo : public KeySystemInfo {
   bool UseAesDecryptor() const final { return true; }
 };
 
-// Returns whether the |key_system| is known to Chromium and is thus likely to
-// be implemented in an interoperable way.
-// True is always returned for a |key_system| that begins with "x-".
-//
-// As with other web platform features, advertising support for a key system
-// implies that it adheres to a defined and interoperable specification.
-//
-// To ensure interoperability, implementations of a specific |key_system| string
-// must conform to a specification for that identifier that defines
-// key system-specific behaviors not fully defined by the EME specification.
-// That specification should be provided by the owner of the domain that is the
-// reverse of the |key_system| string.
-// This involves more than calling a library, SDK, or platform API.
-// KeySystemsImpl must be populated appropriately, and there will likely be glue
-// code to adapt to the API of the library, SDK, or platform API.
-//
-// Chromium mainline contains this data and glue code for specific key systems,
-// which should help ensure interoperability with other implementations using
-// these key systems.
-//
-// If you need to add support for other key systems, ensure that you have
-// obtained the specification for how to integrate it with EME, implemented the
-// appropriate glue/adapter code, and added all the appropriate data to
-// KeySystemsImpl. Only then should you change this function.
-static bool IsPotentiallySupportedKeySystem(const std::string& key_system) {
-  if (key_system == kWidevineKeySystem)
-    return true;
-
-  if (key_system == kClearKeyKeySystem) {
-    return true;
-  }
-
-  // External or MediaFoundation Clear Key is known and supports suffixes for
-  // testing.
-  if (IsExternalClearKey(key_system))
-    return true;
-
-  // Chromecast defines behaviors for Cast clients within its reverse domain.
-  const char kChromecastRoot[] = "com.chromecast";
-  if (IsSubKeySystemOf(key_system, kChromecastRoot))
-    return true;
-
-  // Implementations that do not have a specification or appropriate glue code
-  // can use the "x-" prefix to avoid conflicting with and advertising support
-  // for real key system names. Use is discouraged.
-  const char kExcludedPrefix[] = "x-";
-  return base::StartsWith(key_system, kExcludedPrefix,
-                          base::CompareCase::SENSITIVE);
-}
-
 // Returns whether distinctive identifiers and persistent state can be reliably
 // blocked for |key_system_info| (and therefore be safely configurable).
 static bool CanBlock(const KeySystemInfo& key_system_info) {
@@ -391,13 +340,6 @@ void KeySystemsImpl::ProcessSupportedKeySystems(KeySystemInfos key_systems) {
     DCHECK(key_system->GetDistinctiveIdentifierSupport() !=
            EmeFeatureSupport::INVALID);
 
-    if (!IsPotentiallySupportedKeySystem(key_system->GetBaseKeySystemName())) {
-      // If you encounter this path, see the comments for the function above.
-      DLOG(ERROR) << "Unsupported name '" << key_system->GetBaseKeySystemName()
-                  << "'. See code comments.";
-      continue;
-    }
-
     // Supporting persistent state is a prerequisite for supporting persistent
     // sessions.
     if (key_system->GetPersistentStateSupport() ==