This has nothing to do with other instances. The join-lemmy.org site is run by the Lemmy developers and they decide what happens with that site. They think it’s problematic that lemmy.world is as big as it is (as one of the points of the fediverse is decentralization). So they removed lemmy.world from the listing on join-lemmy.org.
Note that this is in no way a defederation or anything of that sort. The site just doesn’t show lemmy.world, that’s all.