Steam has limited rollback support from the command line which we had to do plenty of times for Starfield when working on Luma. Sometimes updates are small. Sometimes the entire exe gets reshuffled so you have to find where to patch the exe all over again.

All the versions are apparently there. You just need to download the "depot" and it'll dump into a folder. From there you copy that folder over your game directly.

It also works the other way around. I can download the depot for the latest version and stay on the version I'm at. It's useful to pick apart and diff what was actually changed.

Why they can't add that as an option I'm not sure. That seems more of a UX/UI issue rather than a technical one (like avoiding people using old versions on the web server).