Allow me to send a request parameter “?stfu=Y”

Which breaks on search result and referral traffic, forcing users to install third party extensions to maintain, thus creating new security vectors.

Why not just load your own css and set the div to "display:none;" then?

This is what I do on browsers that have greasemonkey. The downside is that I have to be aware of their div and other things they make to try to make me switch to mobile.

Specifying a parameter of reddit’s specification allows me to work with them as a user to follow their parameter. If their users don’t like certain features (like nag popups) then it helps them to proactively solve this problem before their users leave.

Sure, but as a PM at reddit, you're not going to release new features like blanket URL parameters that push behavior that leads to security issues. Just to cater to a self-selecting demographic that doesn't directly contribute to your P&Ls.

Honestly, that's a good idea. I should block the XPromoPopup class with a content blocker.