So I can NOT access old.reddit anymore on any of my linux machines with firefox and nordvpn extension, but I can access both old and new on windows with firefox and extension, and old on android with FF mobile and nordvpn app. Haven't tried new on android for obvious reasons.
Been having issues with Google/Reddit etc. lately as well on Proton, managed to get around it by switching to a new IP subnet. It's likely they've just blocked IPs with malicious users behind them, which sucks for the rest of us.