People are either already on a platform like GitHub, in which case there's an Issues product right there that can be used with much less friction, or they aren't but this idea of "sending a merge request" still feels like anachronism.

Joining mailing lists and configuring your SMTP client not to mangle patches seems like an art that everyone just decided to forget how to do.

(As an ex-maintainer of the Linux kernel I'm aware that they still do it there. Just.. mostly no-one else.)

The Git project still does mail-based patch sets! git originated via Linus for the linux kernel, so that's not super surprising.