Googlebot started scanning sites over the HTTP/2 protocol
On November 12, Google updated the help section for developers (Googlebot Developers Support Page) to show that Googlebot can now scan sites using the latest HTTP/2 protocol.
Google warned about this update back in September.
Site owners can opt-out of crawling over HTTP/2 by configuring the server to send a 421 server response code.
Recall that the HTTP/2 protocol is the latest protocol that allows faster and more efficient data transfer between the server and the browser (or Googlebot).
Unlike HTTP/1, it does not create a separate connection to download each file, but downloads them all in parallel, one stream. Multiplexing allows multiple request and response messages to be transmitted simultaneously; you can even mix parts of one message with another over the network.
Such HTTP/2 features reduce server overload and save server resources. Minimizing the load on server resources is useful for websites. Sometimes, not only Googlebot, but other bots also hit the site at the same time. As a result, the site starts to respond slowly because a lot of server resources are consumed.
You can check if your site meets the HTTP/2 crawl requirements using the HTTP/2 Checker tool.