Replica set members in standalone mode will no longer be allowed to take user writes. Metadata operations will continue to be allowed. The user can override this default with a new server parameter, which will also set an existing flag that will prevent the re-introduction of the node back into its replica set. So writes will be allowed again, but they will cause the node's data to diverge from the replica set, and would cause data inconsistencies if allowed to rejoin the replica set, so we won't allow it anymore.
The new server parameter should cause the index builds found on startup to be dropped, just like a non-repl member in standalone mode does on startup, since retaining them is only important for re-introduction to the replica set.