Are nostr clients really actively filtering posts like that though? I've been under the impression that nostr is bitcoin-centric, so people just gravitate toward that topic.
It's the client devs who have left the impression, that Nostr is Bitcoin-centric. It isn't inherent to the protocol. It was originally focused on free-speech, in general, not just money.
I think its more than just the devs though. Early on, I remember all the bitcoin podcasts were talking about nostr, and around that time bitcoinhackers on mastodon was shut down and people basically moved to nostr, etc. I don't remember anyone caring about nostr but bitcoiners. I don't blame the client devs for that. So we find ourselves in this weird place where bitcoin is keeping nostr active, but its probably holding it back as well.