Misha Collins’ Castiel was actually responsible for Dean’s first blast to the past, and Cas’ time travel abilities mean that he could turn up without contradicting his canon ending in Season 15.
Plus, if Dean is narrating from the afterlife, Cas could turn up (or voice cameo) in that way.
Collins also pitched a scenario for an appearance on the show on Twitter, suggesting that it “would benefit from a time-traveling angel-in-a-trenchcoat character.”
Collins admittedly doesn’t look exactly as he did back when he filmed his first few seasons, but fans have suspended their disbelief for much crazier things when it comes to Supernatural!