As quoted from the linked post.

It looks like you’re part of one of our experiments. The logged-in mobile web experience is currently unavailable for a portion of users. To access the site you can log on via desktop, the mobile apps, or wait for the experiment to conclude.

This is separate from the API issue. This will actually BLOCK you from even viewing reddit on your phone without using the official app.

Archive.org link in case the post is removed.

https://web.archive.org/web/20230611224026/https://old.reddit.com/r/help/comments/135tly1/helpdid_reddit_just_destroy_mobile_browser_access/jim40zg/

  • Reddit doesn’t want to have people browsing from Firefox because it’s one of the last privacy conscious browsers available on mobile. They’d rather block you from using the service entirely than allow you to do what they call ‘freeloading’—accessing the site without viewing ads or becoming infinitely trackable through the use of their data-sucking app.