You have to be logged in and, if I understand correctly, if you don't log in to old.reddit for six months (retroactive) they take it away.
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.