Sounds like somebody gave you some incorrect information re: banning.
You don't need a w3c standard to have a protocol that is open source and used globally, it's just one way to go about that. You can also have standards which are not made through w3c but are made through some other governance body, or you can have standards where the standard just kind of evolves from a bunch of different devs trying different versions of things until there's one main way which floats to the top since everybody prefers it. Nostr has the NIP (Nostr improvement proposal) process which has been used to make standards for everything from video streaming to calendar events/invites.
Relays on nostr, which are the equivalent to instances in ActivityPub/mastodon/lemmy can set their own moderation policies, defederate from other relays, etc all the same as in ActivityPub. The moderation abilities are the same. This means relays can choose what content they allow and ban users/topics/content from other relays, etc. The key difference is that you are by default connected to multiple relays. So if your relay blocks a user you really want to follow, you can keep following that user and see them in your feed, they just don't show up for other users on that relay. If a relay blocks you, you can't post content to that relay. So you get the best of both worlds: relays have curated, moderated public squares with trending hashtags and tweets while not reducing your ability to choose who to follow and who can follow you.
Identity portability is another key feature: if your instance goes down, you don't lose all your DMs, followers, etc.