[1] mod_rewrite - Apache HTTP Server Version 2.5 () https://httpd.apache.org/docs/trunk/mod/mod_rewrite.html#RewriteCond

THE_REQUEST

The full HTTP request line sent by the browser to the server (e.g., "GET /index.html HTTP/1.1"). This does not include any additional headers sent by the browser. This value has not been unescaped (decoded), unlike most other variables below.

[2] HTTP/0.9 や HTTP/2 の時どうなるのだろう。