Hi folks,
So, taking a step back on this issue, I put out a query in various community management/developer relation circles to see if anyone knew exactly what the determining factors were for getting sanctions imposed, and was pointed at a few very large communities using Slack - example https://invite.slack.golangbridge.org/. 44,420 registered users 
This morning, I sent Slack support a message asking directly about this ‘upper limit’ and whether it applies to communities who are on the complimentary Open Source plan.
The response is as follows:
Hi Ruth,
Thanks for reaching out today. While I cannot share specific information about other workspaces, the publicized reports we’ve seen of Free Plan workspaces experiencing issues with an “upper limit” were not using Slack the way it was intended, and found difficulty using the product in a way that supported how they wanted it to work, rather than what it was designed for.
For our customers using Slack as a collaboration tool for work or projects, the product can comfortably support an unlimited number of users.
If you have any additional questions, I am happy to answer them.
I have asked for clarification on what is classed as “not using Slack the way it was intended” and will report back.
While I do agree that Slack has some shortcomings, I think at this point if we don’t have to move platforms, it would be one less thing to cause problems and difficulties with the community at a time where we need to focus on bringing people together and avoiding disruption and confusion.
Feel free to discuss and give your thoughts/opinions!