Starting September 1st, about 25% of crawl requests started returning 304 Not Modified instead of 200 OK. Prior to that, there were just a small handful of such requests per day.
I did not make any code changes that would introduce returning this header. Once I noticed, however, I tried visiting a handful of the pages as a logged-out user (where caching is enabled) and noticed that my Chrome web browser was also returning 304 headers.
We use Cloudflare. Did something change either with Googlebot (making the request) or with Cloudflare (responding to the request) that would cause this change of behavior? I found no articles suggesting that Googlebot just recently started using the if-modified-since header.
Edit: I'd like to add that I have Cloudflare configured to cache HTML pages for logged out users.