A look back into the history of HTTP and how it evolved to its current form as HTTP3: The Long Road to HTTP/3
While HTTP/3 specification is still in the draft stage, the latest version of the Chrome browser already supports it by default. With Chrome holding around 70% of browser market share, you could say HTTP/3 has gone mainstream.
Looks like Apple have added it to Safari in macOS 11 and iOS14. In iOS you have switch it on yourself under Settings > Advanced > Experimental WebKit Features, I’m not sure about macOS. Be nice to see it on by default.