I was curious about your question and so I did a little digging. This data is a little old, but not too old (4 months). At that time, over 100 American universities required a booster to attend fall classes:

https://abcnews4.com/news/nation-world/over-100-colleges-still-have-covid-vaccine-requirements-ahead-of-2023-school-year-report-says-johns-hopkins-harvard-rutgers-coronavirus-pandemic-vax-health-sickness

I suspect that there could be pockets of government jobs here in the states and pockets of Europe that still requires it as well, but I'm continuing to research.

As an aside to the OP/BD, great post and digging your twitter feed as well.