Hi, thanks for the report. There's a decent chunk of Windows binary packages missing right now, and we're working to resolve that (also see Migrating from MRAN to PPPM: thousands of win.binary packages missing. What to do? - #2 by tylfin).
Those listed packages aren't available as binary and are actually source packages (.tar.gz), so I think this is a bug - Package Manager should really be returning 404 responses on those URLs. We'll look into getting this fixed.
In the meantime, this shouldn't affect the usual package installation in R. Those missing binaries aren't included in the binary PACKAGES listing, so R should only be downloading the source packages in .tar.gz format, e.g., from https://packagemanager.rstudio.com/cran/latest/src/contrib/rstan_2.21.8.tar.gz, and never attempt to use those invalid .zip binary URLs.