I bet it depends on the institution and the IT team behind said institution, but at least for my university we apparently don't delete old course shells or anything.

I'm friends with a professor who complained to me a couple times about how sometimes he will need to scroll through pages and pages of courses he taught in the past. He also mentioned that profs aren't able to delete their own course shells either.

It wouldn't surprise me if most of it is still around. The amounts of data are probably fairly small, and thus unless intentionally deleted, it's probably still there (maybe unis in Europe are more likely to bother to click the relevant buttons as to comply with the GDPR?). I can't imagine storage becoming an issue unless you've got a huge uni or classes that deal with video (and even then, those probably end up on Youtube as private videos, or only as really small clips).