As far as I know this isn’t a feature that exists, but I know the protocol should make it fairly easy.
What I’m thinking is for Lemmy to basically have the option to inherit the comment thread when posting a URL to another fediverse service.
E.g. If crossposting a Lemmy thread, you get a tickbox saying “inherit comment section” or something and it makes the new thread effectively a symlink to the original.
This could also be used to bootstrap other fediverse services like pixelfed and peertube by enabling people to comment directly from their Lemmy instance.
@catloaf under the #ActivityPub model, no, if you're banned from one community the ban has no bearing on what happens on instances that don't actively respect that ban.
You can still comment away. The ban merely means your comments won't be displayed to some people.