Re: more acceptable use cases
"neither your nor my addon works on mobile."
Yes it does. Cookie AutoDelete 3.0.2 running on Firefox 60.0.2 for Android. It's what I'm using right now.
"some sites like Medium seem able to track effectively even with"
There are no doubt other methods (that stuff that Chrome refers to as "site data" as distinct from cookies). It's a bit of a game of whack-a-mole. :(