Support the stable prefix for MSC4287 (key backup preference)

src/@types/event.ts

@@ -416,9 +416,12 @@ export interface AccountDataEvents extends SecretStorageAccountDataEvents {
     [EventType.Direct]: { [userId: string]: string[] };
     [EventType.IgnoredUserList]: { ignored_users: { [userId: string]: EmptyObject } };
     "m.secret_storage.default_key": { key: string };
-    // Flag set by the rust SDK (Element X) and also used by us to mark that the user opted out of backup
-    // (I don't know why it's m.org.matrix...)
+
+    // MSC4287: Sharing key backup preference between clients - used to mark that the user opted out of key storage
+    "m.key_backup": { enabled: boolean };
+    // MSC4287 unstable prefix (note the boolean property has the opposite sense)
     "m.org.matrix.custom.backup_disabled": { disabled: boolean };
+
     "m.identity_server": { base_url: string | null };
     [key: `${typeof LOCAL_NOTIFICATION_SETTINGS_PREFIX.name}.${string}`]: LocalNotificationSettings;
     [key: `m.secret_storage.key.${string}`]: SecretStorageKeyDescription;

src/crypto-api/index.ts

@@ -626,7 +626,7 @@ export interface CryptoApi {
      *  * Disables 4S, deleting the info for the default key, the default key pointer itself and any
      *    known 4S data (cross-signing keys and the megolm key backup key).
      *  * Deletes any dehydrated devices.
-     *  * Sets the "m.org.matrix.custom.backup_disabled" account data flag to indicate that the user has disabled backups.
+     *  * Sets the "m.key_backup" account data flag to indicate that the user has disabled backups.
      */
     disableKeyStorage(): Promise<void>;