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.
This doesn’t really make sense in the Lemmy model inherited from reddit. A post belongs to a specific community, you can’t put one post in multiple communities. You can crosspost, and from there you can jump to the original and see its comments, but the comments on a crosspost belong to the second community.
Maybe a crosspost should not have a second comments section, and opening it should always send you to the original post, but I don’t think that’s desirable. For example, if I’m banned from the first community, should I be forbidden from commenting on the post when it’s crossposted to my second community?