On mobile it reverts you every few days whether you want it to or not and you have to set it again.
There are a few extensions around for each browser that fix this, but every time it happens it makes me angry at whatever person thought this was a good idea.
If they rely on metrics for old vs. new this 'feature' obviously distorts the reality.
I think they do experiments, I used to get reverted daily, and so changed all my bookmarks to old.*. And at some point, they removed that behavior, and now remembers my prefs. The day they stop supporting old, I'm out of reddit.
e: I just realised, you may need old.reddit.com if you don't sign in.