Version 2.21.1-1 of r-cran-rstanarm is marked for autoremoval from testing on Wed 28 Apr 2021. It depends (transitively) on r-cran-dt, affected by #985281. You should try to prevent the removal by fixing these RC bugs.
Last update: 2021-04-10
Standards version of the package is outdated.
The package should be updated to follow the last version of Debian Policy
(Standards-Version 4.5.1 instead of