Thanks for the report. This was an issue on the Posit Package Manager side, as our rstan binary builds were failing from running out of memory. The latest version of rstan seems to have a significantly higher memory requirement for compilation than before. We're accounting for that now, and rstan should be installable in both Cloud and from Package Manager, with dependent packages like rstanarm coming soon.
If you come across any other missing binary packages in the future, you can report them on Community using the package-manager tag.