We're known for it, it's part of the interviewing process for Labs and most of R&D. Folks who don't want to pair generally don't apply. Of those who apply, some try it during the interview process and decide it's not for them.
That is a good thing.
Besides which, solo work is common too. Most of the field org works solo, Spring folks mostly work solo, I love pairing but currently I'm working solo. If a pair decides they should work solo, they do. Lots of pairs have a rotating arrangement where somebody different works solo for a day or two. I know of teams where they are solo in the morning to review PRs and pair in the afternoon to work on new contributions.
None of this is enforced. There is no manual, no rulebook for The One True Way. Nobody tells you how to work. The way works is: the people are nice and smart and want to do the right thing. The rest is largely details.