Also it was supposed to be entirely optional, but the current spec editors would like you to believe otherwise.
The part I love about the authors to the spec, is that as far as I'm aware, none of them have even made their own complete implementation in any way, and only one of them has made some contributions to Mastodon and Pleroma.
Meanwhile, one of the authors had their own federated microblogging application from the OStatus days (that OStatus was defined from), that they abandoned entirely, and had no interest to even attempt a remake or a fork with ActivityPub at all, despite both being their project.
I don't trust people that can't even follow through implementing their own suggestions/ideas.