FYI - Next Stan release (2.21) October 18 - now with feature freeze

Hey all,

Just a reminder that we’re doing quarterly releases and the next one is October 18th. I brought up at the Stan meeting today the idea of having a feature freeze for the week before the release and everyone agreed. So I’ll ask folks contributing to CmdStan, Stan, and Math repos to not merge any PRs that aren’t (small) bugfixes during that week. This is motivated by the 2.20 release which had some heroic last minute efforts to get fast compile times in but also broke GPU and MPI and we found out <24h later.