>all popular instances have a very strong left-wing set of guidelines

Can you point to those guidelines on the top instances? I'm not sure which of the most popular instances prohibit, say, expression of conservative views. Though perhaps you do have a point - they might be more tolerant of far-left views than far-right views.

I also think defederation because of lax guidelines doesn't happen as often as defederation because of users of other instances behaving in ways which heavily go against the guidelines - whether it's harassment or posting porn the major instance doesn't want, etc.

Fedi, moreso than Twitter, is what you make of it, and there are many (but just not as many) users on 'lax' instances (e.g. FSE or SPC or Smug or Weeaboo.space or Gameliberty... you name it) to talk to. I have an account on a major instance, a 'lax' instance, and my own instance. I have personally found I extract the least value from the local timeline 'lax' instance.

I'm skeptical of the idea that defederation-by-association is as common as another commenter here suggests.

I don't think it's meaningful to say a newspaper is 'censored' when there's another newspaper stand right next to it that shows every paper in full.