I been gone for awhile. I now know about the MC and buying one for 2,000 CSC but do the plushies still work? Any other ways I don't know of?

No, the old plushies don't work anymore.

https://subeta.net/preferences.php

You can see a pet slot breakdown on that page. The only ways to get them now (that I know of) are buying them with CSC, the Loyalty Box option, the Millionaire Center, and the occasional event prize for things like quest-a-thons.

Mystical plushies don't work anymore.

Currently, pet slots are now given from the MC, CSC (through the pets page), loyalty totems, and slots given from 'awards and prizes' such as the Quest-a-Thon mini event.

Thanks guys! I had no idea they showed it like that on your dashboard. Is the "new slot additions (max 30)" from buying them at 2,000 CSC?

Yes, I'm pretty sure that's what that line is referring to.