Channel#current() uses thread-local storage to determine the current channel. It returns null if the channel does not exist Some writeReplace/readObject/etc. serialization logic retrieves the channel in order to export the object via ExportTable. Obviously, such operations will fail if we try to serialize the object without Remoting context.
Full list of classes to be updated is TBD, some of them are referenced in https://github.com/jenkinsci/remoting/pull/206