A customer asking you to join their slack seems like an outrageous ask on the face of it. Every slack channel joined is a fairly heavy additional burden. I'd say "no" as well.

You'd be surprised how common of a request it is!

That's amazing. I've yet to get that request (I get the opposite -- customers wanting to join my slack channel -- which makes more sense to me).

If you run a digital product, having a Slack/Discord is a good way to let people get help, it makes a lot more sense.

Six years later, we still haven't joined your Slack.