That stopped working for me on my mobile firefox (but not on my desktop) literally yesterday. It's pretty clear they're doing some kind of A/B testing.
I keep reading this about old.reddit.com and yet I keep using it without being logged in. It must be some kind of A/B testing or slow rollout? Or maybe it's regional?
Ironically enough, reddit recently killed off old.reddit; you now have to log in, which defeats the point of old.reddit.com totally.