I really like this slack alternative which integrates well with telegram and signal.
The mobile app seems to be well maintained and the protocol is open.
I really like this a public comms channel for a local chapter or event, eg FOSS4G.
Personally I find mailing lists rather difficult to engage with and was wondering if people would like to see a public OSGeo room or whether other Local Chapters use Matrix / Element as a member / delegates comms channel.