new users should always understand what they’re signing up for.

I agree that defederation is a feature, but this in specific is a contradiction against what is the normal "it doesn't matter what instance you sign up for" that gets said whenever discussion about onboarding new users for Lemmy, Mastodon, etc comes up.

I run my own instances for both (partly to avoid this, and also because I quite enjoy self-hosting) but if I didn't and wanted to get my friends onto the Fediverse, this would be something I'd have to take into consideration for them as I couldn't expect them to possibly understand it.

In reality, its more like "It doesn't matter what instance you sign up for... unless there is someone/a community you want to follow on instance X which is defederated by instance Y and the only way around it is to either create multiple accounts (which most non-tech oriented people aren't going to want to do) or pick a different instance as your main instance and manually move everything over since there are no migration tools for the most part".

In my opinion this is probably the number one reason why the Fediverse will never be mainstream, for better or for worse (though I'd be incredibly happy to be wrong about this).

Again, I think defederation is a tool that needs to be present on the Fediverse for pretty obvious reasons but I do think that it is also a double edged sword.