Retention would not be requested because he isn't worth it, but because there aren't many teams out there with 6M cap space right now. I would love to have him, but we need more space to add anyone, let alone adding a 6M player at a position that isn't our weakest spot. Don't take it as a shot on the player, just of the current state of affairs when it comes to trading mid to high price point players.
I would think the Isles would be willing to take back a soon to be UFA's ugly cap hit, allowing the buyer to deal with any cap issues over the course of the summer and FA period.
That being said, I don't know why we'd want to trade our only other offensive threat than Barzal.
Beauvillier and Bailey would be available but they'd both require retention and/or significant sweeteners.
We're basically screwed til the 2023-2024 offseason as far as the contracts to Bailey, Beauvillier, Clutterbuck, and Martin go. Trading Nelson as a stop-gap solution would be a seriously awful thing to do, especially since we have a lot of cap space this year and next (assuming Lou doesn't offer Mayfield a contract).
I don't even think I'd want to go down the rabbit hole of who will be available as a UFA in July of 2024 since a lot of the players of the type the Isles need will be resigned...