In order to avoid this, what would you think of having a "new joiners" instance, where
hexbear, lemmygrad and ml would be defederated
politics and news communities would be blocked at the instance level
That could help to onboard people, so that the first time they look around, they see more gardening, cute comics and casual conversation rather than another set of depressing memes.
Disclaimer: politics and societal issues are important and should be discussed extensively (they are quite popular on Lemmy, let's be honest). I'm not advocating to hide them all, just to not show them as the first content people potentially interested in Lemmy would see.
The onboarding issue isn't politics or which instances are federated, it's that federations exist for all to see when it's something that should impact the server side only and users should come to Lemmy and feel like they're joining just any other centralized website.
they’re joining just any other centralized website.
That's not the case. Users have to attach themselves to an instance or another, and the content they will be able to see will change accordingly. That's how ActivityPub works