This already happens now. Go look through a few of the "Show HN" authors - you'll inevitably see around several accounts that are 50-100 days old with a karma of 1 to avoid a green label.
The OP is talking about posts, not comments. The simplest solution might be to prevent someone from posting a "Show HN" until they’ve earned twenty-five or fifty karma, to demonstrate that they’ve been actively participating on Hacker News rather than using it solely to promote themselves.
Yeah I considered that - but any friction is better than none. Maybe integrate an additional consideration by which low karma (threshold < 5 karma) accounts cannot upvote other low karma accounts.
Honestly, we don’t really have the same cold start problem that a brand new social media site would. We already have plenty of reputable active users here. So HN could restrict new accounts to only being able to comment initially. As they participate, their comments receive upvotes, allowing them to build up enough karma (even a small amount of 25) which unlocks the ability to upvote, and then, finally, the ability to create posts.
Creating more friction can also lead to a higher percentage of bots. I for one immefiately leave when I realize that I need to jump through several hoops before I'm actually allowed to participate on a site. Someone building a bot farm on the other hand is probably willing to tolerate quite some friction before giving up.
I have seen accounts that were dormant for years suddenly start posting frequently, all with slop. (I don't know if this represents people having an epiphany about AI use, or accounts being compromised or just what.)
The OP is talking about posts, not comments. The simplest solution might be to prevent someone from posting a "Show HN" until they’ve earned twenty-five or fifty karma, to demonstrate that they’ve been actively participating on Hacker News rather than using it solely to promote themselves.