Both Google & Bing Support ETag/If-None-Match

Google's John Mueller said Google does support ETag/If-None-Match after this Search Engine Land article shows that Bing supports it. He said it is rare to see sites using it, most use If-Modified-Since instead.

John Mueller when asked if GoogleBot supports that ETAG, said last month on Twitter "Last I looked we supported that, though most sites seem to use If-Modified-Since more."

Last I looked we supported that, though most sites seem to use If-Modified-Since more.

— 🍌 John 🍌 (@JohnMu) November 13, 2018

Mozilla has a more technical description of this header.

Forum discussion at Twitter.