For me and most of my friends, Slack has filled that gap - but more for groups of friends, with overlapping subsets of interests. (e.g., I'm in a NYC redditors slack team with 52 people, and we have over a dozen channels.)
For smaller interests, a lot of people tend to use Google Hangouts - I'm in one with mostly people I know focused on the game Star Realms.
Slack's harsh message retention limits and steep slope from free to paid surely are going to limit it's reach for many communities that a) can't persuade participants to pay £5/month and b) grow to the point that 10000 messages means almost no retention at all.
It's happened to more than two Slack groups I've been part of and they are both looking for a new platform.
Hmmmm. As much as I wish Slack would become more attractive to non-commercial communities - Spaces doesn't even come close. No integrations, no @mentions, none of the UI niceties, no 'presence'.
I would imagine that for a lot of small groups, lack of retention is just fine. If I'm chatting with friends about playing Star Realms, I don't need to see what was said two months ago.
For smaller interests, a lot of people tend to use Google Hangouts - I'm in one with mostly people I know focused on the game Star Realms.