Having used slack for over 3 years we are running into some issues, as @avehtari formulated it in the slack environment:
This Slack workspace is using the free plan which has limit of 10k messages. In some channels we have been and still are losing useful information beyond the paywall, SGB checked that we can’t get non-profit discount from Slack and regular plan is quite expensive. The suggestion to move to Zulip that provides free plan for free software projects.
You can join the Stan Zulip here
You can find the related discourse thread here.
Update: We have been able to get an upgrade for Slack which should resolve any current issues. Both channels will remain open. Anyone is welcome to join Slack at mc-stan.slack.com.