To be fair turning him into a blue fart is probably better than what would have happened if the Shadowlands writers had actually tried to write him into the story somehow.
No need to retcon it at all. Just have the screen fade to black after you kill the Jailer.
You wake up in the Chamber of the Heart, and Magni is the “Defeated Jailer” turn in for the final quest.
"Champion! Yer back! Been almost a month since ye was laid out from N’zoth’s mind control! We feared we’d lost ye with those fevers ye had! What…? Shadowlands? Sylvanas? Oh no, no, no, Sylvanas an Nathanos were killed by Saurfang and Tyrande at the Marris Stead. Their heads be outside Orgrimmar an Darnassus respectively. Saurfang’s the Warchief now! What’s the “Jailer”? Is that some new thing they do in Goldshire? … …Champion, I think yer head still be swimmin. Why not rest a while longer?"
Stories aren’t supposed to end exactly the way readers/viewers want. Not unless Hallmark movies are your thing. In that case it’s always the happy ending everyone wants. Stories that always end the way you want are a ticket to boredom.