There's a libpurple plugin, so you can in theory use Teams chat from Pidgin, finch, or other clients. I tried it last year and it didn't work very well.
The graph API gets you 60% of the way there, but sadly, some core functionality is missing, like calls. And even the chat part of the API is throttled aggressively. If the teams graph API were better, we'd have a bunch of nice, lightweight 3p teams clients by now probably. I really wish this were the case, given that I find teams regularly using multiple gigabytes of memory. Even on a beefy dev machine, that's significant. The C++ linker runs out of memory sometimes even on my 96GB workstation, which fails the build. No kidding, the internally documented solution is to terminate your memory-hungry apps (such as teams, edge, vscode) and try again. Or bump your page file size up to something crazy like 100GB, which ends up cutting into the already tiny 250GB C drives we've provided with. Which means your C drive will fill up every 3 weeks and you have to go babysit it by deleting all the bloat folders.