Not sure if it's just me, but that setting seems to frequently get reset, as I tick that box maybe once a week/two weeks or something, but I keep getting sent to the new reddit at one point. Maybe I accidentally access new.reddit.com which forces the new layout and then reset the setting?
In any case, I'm giving one of the new -> old redirection extensions a try now instead.
It doesn't apply to mobile but IMHO the new site is slightly less terrible on mobile than the old one anyways so that's my preference.
e.g, you want to browse for a gift for your SO and you're review-hunting, but you don't want it showing up in your sidebar as a recently viewed thread, or being in your history, or whatever - so you don't sign in. Userscripts/extensions force old.reddit.com regardless.