Gandi.net is the registrar whose freedom-of-speech record I’m most aware of being positive. I looked just now to see what kind of written guarantee they provide, and couldn’t find one, but their reputation on this is pretty good. The article https://www.cnet.com/news/survey-are-domain-registrars-free-speech-friendly/ is from 2008, but it’s the closest I could find to non-anecdotal evidence. It also mentions DirectNIC as a standout.
FWIW, I have no affiliation with nor financial interest in Gandi, other than being a customer of theirs, and receive no benefit for shilling for them :-). I agree with you that free-speech-friendly registrars are important, though, and Gandi was my choice in part for that reason.
(The other reason can be seen here: https://www.gandi.net/en/gandi-supports .)