We try to keep requests and responses as-is as much as possible. Compression seemed like something we should do to prevent thousands of apps implementing their own.
Hmm, you might still want to set the Vary header here. We’re not caching anything, but if something upstream does, then it should vary by content-encoding.
FYI, if you’re considering promoting this to a supported feature, an example of how others do it: The folks at Cloudflare key off of Cache-Control: no-transform to disable features like gzip, image minification, etc (their docs).
(We found this out the hard way when their frontends were stripping our etags, a consequence of one of those rewriting features…)