It's not that it is outdated but that right now sphinx version is unconstrained, so using the latest sphinx version should be fine. I agree it can be a bit confusing if you are looking for an exact version.

Linking to the lock file could be an option, as I mentioned in #27643 (comment). I just realised that regex is supported by github search so the following link would be clear:
https://github.com/search?q=repo%3Ascikit-learn%2Fscikit-learn+%2F%5C%2Fsphinx-%5B0-9.%5D%2B%2F+path%3Abuild_tools%2Fcircle%2Fdoc_linux-64_conda.lock&type=code

Which gives a single line with sphinx + version highlighted:

Originally I was not sure that linking to the lock file was worth it, but using the link with the regex above, I now think it is, so PR more than welcome!