It looks like old.reddit.com still works but requires logging in. Not having to log in was a huge feature for old.reddit.com. Sucks to see it go, but it was inevitable.
So basically, I probably have a bunch of accounts from years ago; suspect that all but my current one no longer work with old reddit.
Biggest annoyance for me would be on mobile where i wouldn't log in.
>Use new Reddit as my default experience (by enabling this, you will be redirected to the new site when you go to any supported https://reddit.com page)
Undid that and it fixed it for me.