Well, there’s also IRC and XMPP. Could those work better for older devices? Dunno, but neither are encrypted, so don’t use them for anything sensitive. Signal is good if you want security, they’ve been audited a lot, but they are based in the U.S. which isn’t great.
I’ve also heard of Stoat (formerly Revolt) and Fluxer, both are relatively new, but I would prefer Matrix over both since it’s more easily self-hostable (+ all those different clients to choose from!)
edit: I really didn’t know, but looks like XMPP does support encryption! (and it’s enabled by default on most clients!)