schmunk
Why oh why oh why?
Well, such a clause is not enforceable - it contravenes the rules. But if Palace say (informally, of course) that they'll only sign off on the deal, on condition that he doesn't feature, then Everton would go along with it, to get it done.
Given that the transfer would be likely less than 48 hours before that game, and Wilf hasn't even been introduced to, let alone trained with, his new team-mates, it isn't at all a big concession for Everton to make.
Could Richarlison play in a Zaha mask? Similar angle of play.