Recommend you use the Redirector browser plugin (https://einaregilsson.com/redirector/) and create a rule with these options:

Redirect:

https://*.reddit.com/ to:

https://old.reddit.com/r/$1 excluding:

https://old.reddit.com/ Example:

https://*.reddit.com/ → https://old.reddit.com/r/* Applies to:

Main window (address bar)