This thought crossed my mind when reading your post too, but the cookie set when someone logs in is a session cookie, so it’s gone after closing the browser… there’s a list here: Privacy policy | Discourse - Civilized Discussion — we have some longer-stored cookies for themes, but most users probably won’t have one set.
So we’d need some other method, maybe a separate cookie without any personal info that is just tells us if someone has logged in before? otherwise, yeah maybe a query parameter for people coming from email?