My 0.02€:
- Gitter is dead
- Slack is always open here, since I can not get around
- Discord is always open here, since we use it at BlueDynamics Alliance as internal tool. It's just the best Sound quality of all tools I know.
- Jitsi established as a tool for sprints: Low entry barrier (no login/account needed)
That said, I am not a big fan of Slack, but it just works. Meanwhile it got established and is owned by Salesforce, which might be a good sign.
Explaning Slack to young people is like, its Discord for the boomers. So yes, if you want to address young people Discord is a better choice.
If I had to vote, I would vote for Discord: Almost all Slack offers, including bots. Plus excellent voice and medium (720p, except we pay for it) video/screen sharing plus excellent streaming possibilities if needed.
But as already brought up: the decision is up to the community.