Of course the aprent post might still be deleted after the reply was
already created, but in this case the reply will still show up as a
reply and be federated as a reply with a reference to the parent post.
If the parent was already deleted before the reply gets created however
it used to be indistinguishable from a root post both in Masto API and
ActivityPub.
From a UX perspective, users likely will like to know if the post
they’re replying to no longer exists by the time they finished writing.
The natural language error will show up in akkoma-fe without clearing
the post form, meaning users can decide to discard the reply or copy it
to post as a new root post. It seems sensibly to for other clients to
behave like this too, but so far no more clients were actually tested.
Furthermore, this used to allow replying to all sorts of activities not
just posts which was rather non-sensical (and after all processsing
steps turned into a reply to the object referenced by the activity).
In particular this allowed replying to an user object by specifying the
db ID of a follow request activity (if the latter was somehow obtained).
Note: empty-string in_reply_to parameters are explicitly ignored since
45ebc8dd9a to workaround one buggy client;
see: https://git.pleroma.social/pleroma/pleroma/-/issues/355.
It’s not clear if this workaround is still necessary,
but it is preserved by this commit.
Resolves: https://akkoma.dev/AkkomaGang/akkoma/issues/522
d635a39141