That wouldn't fit the business model of "push them to the native app as hard as possible"

This doesn't work for people who browse Reddit without cookies or signing in, such as for those who use Private Browsing.

So what? That demographic self-selects to be hard to track, so there isn't any good way to track their decision on the pop-up regardless.

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.

You can do that, it's just a bit hidden away. https://i.imgur.com/qIDu7q3.jpg

Of course. Looks like it's stored server side? I couldn't see where they are storing it. Good find.