Hm, that is strange, they have an upgrade process for the past 3 or 4 releases (presently done through the GUI), and they've had unofficial upgrade procedures for versions before that. I've recently got my hands on a notebook I had installed 2017-ish, and got it upgraded to 20.3 with a couple of "next-next-finish"-like wizards.

More here: https://linuxmint-user-guide.readthedocs.io/en/latest/upgrad...