We can see from Fediverse that the spec still isn't really good enough. You have to code against Mastodon's code and Akkoma's code and GoToSocial's code. If you code against the ActivityPub/ActivityStreams spec, it won't actually work in the real world.

Same with most protocols. HTTP is followed closely enough, but forget about even strictly spec-compliant SMTP. Protocol specs are more like statements of intent.