Originally we were on IRC. As the team grew, more CTFs were being played - often concurrently - so having dedicated (private) channels for a given CTF and "sub" channels for its challenges gave us a lot more flexibility.
That being said, we've considered migrating to Discord, Zulip, or Matrix - just haven't gotten around to it yet.
Please consider using either Zulip or Matrix from the get-go, else if you just pick Discord you will find yourself in the very same situation as with Slack before long.
I was in a little CTF server which got flagged and taken down, even though there wasn't anything too particularly nefarious going on. Maybe someone did upload something dumb which raised flags on Discord's end, though.
That being said, we've considered migrating to Discord, Zulip, or Matrix - just haven't gotten around to it yet.