Others have said already, but XMPP and RSS. Also, nobody mentioned NNTP yet.
I wish everything was accessible by NNTP and we had better NNTP clients. NNTP is like RSS but for forums (so, Lemmy, Reddit, or anything where you could reply to posts).
Download for offline reading, read in your client, define your own formatting, sorting, filtering, your client, your rules.
If Lemmy was accessible via NNTP, I could just download all posts and comments I'm interested in and reply to them without any connection, and my replies would get synced with the server later when I connect to WiFi or something.
Probably it would be better to edit my comment, but I'll go with a reply to myself.
To all fans of RSS: there's this service called FeedBase that is essentially a RSS to NNTP gate. You add your RSS feed to that and it becomes a newsgroup on their server, and you can subscribe to it using any NNTP client. New articles appear as new posts in that newsgroup and you can post your own replies to them. So, you get RSS but with discussions or comments.
Back in the day I was a big Usenet fan. What's the modern solution to the spam issue? At the time, folk wisdom was that the demise was being caused by spam, and that due to the nature of the protocol it was somehwhat unsolveable.
I also wonder to what extent activity pub is the barrier to offline use? For reddit, the Slide client had offline reading and iirc posting. I have been disappointed it isn't available for Lemmy. My guess has been it simply isn't a priority for the devs. Maybe eventually we will get it.
I think it would be cool if RSS got put into Lemmy clients. Example you could make a unified inbox for all accounts by automatically getting the private RSS for incoming messages for all logged in accounts. I have manually set this up a couple of times but its tedious. Completely lacks smoothness when it comes to clicking a link, replying etc. But a client could add a little finesse to fix that.
Content addressable protocols are better for asynchronous use. I'd like to see a proper bluesky atprotocol fork with "post lexicons" properly adapted for forums, they're built on top of content addressing and public key based account IDs along with 3rd party moderation tooling support integrated and custom 3rd party feeds/views.