Is Dean not immortal with just the mark of Cain?

But then how did Metatron kill him?

I haven't seen seasons 10 или 11 yet, but my guess would be either a special weapon или spell, или Cain isn't really dead. или are Ты talking about Dean at the end of season 9? He wasn't fully immortal in the sense of being impossible to kill, but he could easily return as a demon, as we saw.