Parlainth, the Forgotten City, of the Earthdawn setting.
Any reasonable guesses as to where (in Europe?) its ruins
might lie?

And has Parlainth ever been mentioned in Shadowrun cannon?

AFAIK, Parlainth has not been mentioned in SR canon, apart from some novel which mentions archaeological diggings in the Ukraine.
When comparing the ED map (or the map in the Theran Empire sourcebook) with real ones, I'd say Parlainth lies north of the Caspian Sea (Aras Sea in ED) and northeast of the Black Sea (Death's Sea in ED), probably somewhere between today's Wolgograd and Orenburg.