I’m not a dreadfully active member of this community… so my words may not have an awful lot of weight, but…
In general, I see Slack as being a “for working thing”, and the multitude of other IM systems (and here I include Telegram, Matrix, IRC, Signal, Threema, etc.) as being “not for work” thing.
I have a personal preference towards Matrix, because it’s a decentralized system (so, if you want to run your own node, you can) with an established and generally stable core server (matrix.org). The IM supports End-2-End-Encryption for all devices in the group, and can (on demand) bridge to other chat environments (like Freenode, and slack, etc)… BUT. For some reason, people don’t seem to like Matrix (in particular, I recall @sil being vaguely not positive about it). For that reason, (and because pretty much all of the other podcasts that I listen to that have chat alongside use it), I’ve backed Telegram. As such, I’ve joined the chat there.
@jonobacon and @jeremy if you do end up setting up the slack as a preference (because that’s where more of your conversations occur… also a fine reason to prefer something like that), please add me - firstname.lastname@example.org