The group I currently play with uses a group chat over SMS/MMS. But I'm going to be DMing a campaign with an open table, so I need something more persistent and flexible. I've used Discord in the past but IMHO it has way too many features.
What have you used to keep in touch with your players?
My group does discord, we play on VTTs so built in voice chat was a requirement. You really don't need to engage with any features you don't want/need to. If you're the only DM you can be the server owner and leave everyone else a default user.
For a more FOSS answer I think Matrix would cover basically everything that discord does. I haven't used it that extensively but it seems to be the best discord alternative when dealing with larger groups where you may want several text channels.
Discord. I got a channel for absentees, one for scheduling, one for talking, one for roleplaying, and of course vocal and webcam features. Also uses a bot for music on it for free. You can add how many channels you want for : lore, maps, links, memes, etc
Groups 1 and 2 are small groups made of friends and we handle everything over text. We do tend to have a lot of "hey, are we playing today?" texts on the day of.
Group 3 started as a work game and has a larger group with less consistency of who shows up week to week. We use discord. Creating events, hosting lore, and places to stash special/homebrew rules is nice.
Group 4 is a west marches-ish game with about 10 players. Discord for that also, though once a group is set for a couple sessions we switch from the discord channel to a group message for communication.
I've used Teams in the past as well but I find discord to be better for managing the various things I need to organize a game.
We use discord. My group is actually about 4 groups connected together with overlapping players, so it's a place we can all congregate virtually. We have a channel for scheduling, ones for in-game content, and some for just messing around.