No, it was more broad than just SE domains, but trying to detect whether a site is suitable for passkeys support automatically is quite the challenge. Some don't allow themselves to be crawled so require manual verification.
I can well believe it is a lot of effort! Just trying to figure out if Oxford had a login took a while to discover it did not - at least, not on that domain.
It seems that Oxford people use SSO to log into a different domain entirely, and the same for applicants.