If you just want to see if an old version fixes your bug, you might not need the source code, rather the installer. You can find those here: Older Versions of RStudio - RStudio
Now if you click on "Installers" for one of these versions, say 2021.09.2 Ghost Orchid, you find the installers for each system. If you click "details" on the right, that opens a table, where the bottom entry is the "Commit Hash", for example for Ghost Orchid:
fc9e217980ee9320126e33cdf334d4f4e105dc4f in rstudio/rstudio
The fc9e21... is the GIT REVISION (=commit) HASH, and by clicking the link you can get to the state of the source code by the time that version was released. You can change commit to tree in the URL to browse it.
Note that on each version's page, there is a link at the top, e.g.
2021.09.2+382.pro1
2021.09 Ghost Orchid
Built on January 4, 2022 (5 months ago) from 481a1f05 | Admin Guide
This version of the hash (481a1...) appears to be broken.