It's a bit overblown if there's no active pandemic thanks to herd immunity, but I wouldn't be against it (if it was a work policy, and not a law)

There is basically no community spread of measles currently due to good herd immunity (via widespread vaccination). When there are outbreaks it is usually very serious and stuff gets closed and people isolated.

Everyone where I live gets the MMR vaccine in grade school.

I also don’t live in Brooklyn, but some people do

For me, it's Canada:

"Routine childhood immunization: 2 doses of any measles-containing (MMR or MMRV) vaccine. The first dose of measles-containing vaccine should be administered at 12 to 15 months of age and the second dose at 18 months of age or any time thereafter, but no later than around school entry."

https://www.canada.ca/en/public-health/services/publications...

Many school boards require it, along with DPTP, hep-b, and others. Maybe it's a socialized medicine thing?

No, it’s free in the US too. There are clusters of families in communities that reject vaccines however, so there are a bunch of schools with insufficient proportions of vaccinated kids. A ripe breeding ground to create a vaccine resistant strain of measles or mumps or whatever.

Vaccine resistant measles does not appear to be possible using standard gain-of-function methods on the vaccine strain, mumps I have no clue. People's vaccines wearing off (or not working in the first place) and them not noticing due to herd immunity would be a much bigger problem, I think.