I saw the "Press /" using Thorium (a Chromium fork). Tried to use the element picker in uBlock to get rid of it. This failed, of course, because the thing to be blocked disappears as soon as the text input area loses focus, even after I deleted every event listener that I thought could have been relevant.

In a Firefox fork (Floorp) using another Google account but with the seemingly relevant part of Gmail configuration matching, I don't see "Press /" at all. Not sure why exactly, it's a somewhat interesting question but I already went poking through dev console to write a selector for it and I'm starting to get irritated at the idea of spending any more time on it, lol

uBlock:

  mail.google.com##div[aria-label="Message Body"] span[contenteditable="false"]

or user styles / Stylus / etc.:

  div[aria-label="Message Body"] span[contenteditable="false"] { display: none !important; }