You can still use old.reddit, you just need to be logged in. That said, if old.reddit ever went away altogether I would stop using it too.

Maybe RES (or other extension) should gain a mode where new reddit is rendered in the style of old reddit?

For you to give that good-good user tracking data you do.

I'm able to browse old.reddit.com without logging in right now.

Maybe it's for specific subs?