Yesterday I found Interstellar which has this feature. I assume mainly because it is not only a Lemmy app, but also a PieFed one, which has this feature in the reference frontend (AFAIK).
Are there other Lemmy apps or frontends that support thread merging?
It’s basically a feature that shows comments from other communities, when there’s a crosspost.
So say, when there’s a link to an article on www.omgubuntu.co.uk and it’s posted to https://lemmy.ml/c/linux, https://programming.dev/c/linux and https://lemmy.linuxuserspace.show/c/news, you can see comments from all three communities instead of having to check each of them separately to get the whole discussion.
(I think it only shows crossposts from federated instances. Or maybe only from the communities you have subscribed to? Not sure)