No you can't, it will pretend to accept your preference and then randomly reset it after a few days.

Weird. I've been using it for months with non-issue. Another comment suggested that maybe it doesn't last on mobile?

Anecdotally, that has not been the case for me. Logged in from any device, I see the old reddit.

For me on Safari, if I leave that tab open, ok, old for awhile. But if I open a new tab, I get the new garbage regardless of my “settings”.

Only option left is to find a content blocker for iPhone that auto-redirects www to old.

It’s really awful, but I’m hopeful these anti-patterns and other Reddit issues (because what is a preference cookie anyhow!?) accelerate alternatives.

I've used it for a couple of years without it resetting

That works on desktop, not on mobile.

Sure it does. I’m defaulting to old reddit without old in the url while browsing on mobile.

Yes, but every couple days it switches back and you have to “request desktop site”.

And it forces a super wide layout that requires either zooming out microscopic font size or scrolling horizontally for every line of text.