Yep - looking at the Lemmy source code, posts have a "read" status.
Lemmy
Everything about Lemmy; bugs, gripes, praises, and advocacy.
For discussion about the lemmy.ml instance, go to !meta@lemmy.ml.
From my understanding, Lemmy is software, while ActivityPub is the underlying protocol. Mastodon, Pixelfed and Kbin are other softwares running with the ActivityPub protocol.
Jerboa has the setting you request under Settings > [username] Settings > Hide Read Posts
That must mean ActivityPub tracks read status and provides it. Just look deeper in the settings.
Sorry, I don't know about Liftoff.
Thanks for pointing out the read settings on Jerboa, it works. And it affects on Liftoff too, those that I read on Jerboa won't appear on Liftoff anymore, but Liftoff does not make read status from within it app.
Some developers were asking whether it was acceptable to use that API call since it would be a lot of server traffic (especially if scrolling past counts as read, which is an option in connect) - so if apps don't all support this feature yet, that might be the technical discussion they're trying to figure out.
Lemmy, being part of the ActivityPub fediverse should have that capability, if not now, then real soon. Remember that Lemmy is still quite young and growing. I foresee great things for it in the future.