The webrequest table has a bunch of ch_* fields; the docs indicate that these should capture the relevant client hint headers (e.g. Sec-CH-UA should be captured in ch_ua). (The documentation mentions the three low-entropy fields and refers to T299397: Measure user-agent client hints already sent in browsers requests; the table schema includes all the other fields that we record for Checkuser, as well.)

This doesn't seem to be actually happening. After T395185: Consider enabling client hints on auth.wikimedia.org every auth.wikimedia.org request has client hints, but there are zero entries with client hints in webrequest.