It never really used ActivityPub Client To Server. See FEP-proposal C2S: websocket endpoint for the first necessary modification. Nowadays, no idea. I stopped caring about trying to follow the ActivityPub specification at the end of March, see bovine/specification_template.md at main - bovine - Codeberg.org.
Furthermore, I’m pretty confident that if I further evolve the software to do what I want. The remnants of ActiivtyPub Client To Server support will disappear. Probably also of ActivityPub Server To Server in some sense.
There are reasons, I like talking about the Fediverse rather than ActivityPub. Mostly, ActivityPub in its current form, is not a specification future generations of servers should follow. ActivityPub is too broad in scope. ActivityPub doesn’t have clear boundaries of what is what making modular implementations unnecessarily hard.
Anyway, I consider pixels a perfectly fine Fediverse application. Creating it using Client to Server would be a nightmare.