We have been running a community slack where you can get your invite via a simple webpage. There are solutions for that.
This is only feasible for free slack which quickly becomes unusable if the user base grows too large (because paid slack requires paying a fee per user, per month, so unless you have unlimited funds or people are willing to pay to join, you can't open it up to everyone). In fact early in its history Steem had such a free slack and that is exactly what happened.
I don’t buy this excuse, but hey, it’s your paid Slack, your rules.
None of these are my paid slack.
But I can tell you what I said is exactly what happened to the public Steem slack a few years ago, and the community was much smaller. It isn't feasible.