It's like toothpaste out of the tube. No getting it back in.

Because you aren't posting to a single, centralized, server, deleting anything that has already gone out to multiple relays is a very finicky thing to accomplish. Hence, you can request it, but it's not likely to be done by every relay that now hosts your note.

Centralized services have lulled us into thinking we can delete things whenever we like, but the reality is, once you put something out on the web, there's no taking it back, even if it appears you can delete it.