I think you can get rid of SYNC_UNLINK_REQUEST, sync_unlinkfiletag, mdunlinkfiletag as these are all now unused. Are there any special hazards here if the plan in [1] goes ahead? If the relfilenode allocation is logged and replayed then it should be fine to crash and recover multiple times in a row while creating and dropping tables, with wal_level=minimal, I think. It would be bad if the allocator restarted from a value from the checkpoint, though.
[1] https://www.postgresql.org/message-id/flat/CA%2BTgmoYmw%3D%3DTOJ6EzYb_vcjyS09NkzrVKSyBKUUyo1zBEaJASA%40mail.gmail.com
