Wow. Most companies at least have the grace to allow users to opt out of these kind of experiments via a "labs" kind of configuration.
It was used during the redesign where they added thumbnails like 14 years ago, to allow people to see the new design before it got launched.
This was the birth of /r/beta
The subdomain is basically the same as old.reddit now, but I continued using the domain all this time for no reason.
The /r/beta subreddit was repurposed for beta testing, but has been abandoned for at least 8 years. I don’t know why they leave the subreddit active.
The subdomain I can only imagine was totally forgotten.
No self-awareness that running such a test ya know might make you the bad guys though.
They accidentally tipped their hand with this. Everyone suspected, but it's basically confirmed that they're pushing to get mobile users onto the first-party app.