As of version 102 I intend to remove IDBDatabase.createMutableFile and IDBMutableFile/IDBFileHandle/IDBFileRequest. They have only been supported by Gecko.
Bug to remove: https://bugzilla.mozilla.org/show_bug.cgi?id=1764771
Pref: dom.fileHandle.enabled
Telemetry shows 0% use of IDBMutableFile.open(). (I separately confirmed IDBDatabase.createMutableFile also has 0% use.)
As the deprecation message says, support for the equivalent feature will likely be added as File System Standard.