@atomicpoet @maegul and ultimately, this is a limitation that makes the platform more important than the protocol, because the platform is responsible for actually accepting the objects: you can't write a separate client that knows what to do with a Tombstone and have it display those objects that were sent to your Mastodon account. So platforms that want those Tombstones to be shown on Mastodon will end up sending Notes instead.
@atomicpoet @maegul again, this might not be considered an AP limitation, but in this sense, Mastodon's dominance is actually destroying that same flexibility of the protocol.