The PIN for being in a Plex home is still available.

What is being referred to in the OP is the easy login method of getting a 4 digit code when attempting to login on a STB device (like a Roku/Roku TV, Android TV, etc.) and then going to https://plex.tv/link and logging in with your account from a web browser and then entering the 4 digit code shown on the TV screen.

I just checked on a Roku I use on occasion for travelling and made sure it was on the latest Plex app and the fast user switching (where you enter a 4 digit pin to switch to a different user within your home).

It sounds like you got logged out for some reason and need to get logged back in, which could be a PITA without the option to sign-in with a code. However, once you get signed in, you should still be able to use the PIN for fast user switching.