Did Umtiti refuse a loan or a transfer? Termination of a contract if the player doesn't play a whole season doesn't protect the player, you've got to look at it both ways. If the player was available and wasn't fielded, then it's not their fault.
Multiple times according to the Catalan rags, "feels he is back to his best and ready to prove himself", "doesn't want to move to a team outside the CL", "Rejected a move to Benfica". Whether or not it is true, the fact remains that he has not left.
I'm not sure about the legality of a unilateral termination clause. Would have to be structured in a way that in a sporting context is proof of not at a satisfactory level to perform. For example what if it is solely based off of league performance and can be unilaterally activated by either player or club if the player does not complete 5% of the compulsory league minutes.
That's just 170 minutes in a league season. So last season
Riqui Puig - 471 minutes
Braithwaite - 236 minutes
Umtiti - 90 minutes
From that bunch only Umtiti could be terminated.
Clearly you got to be a disaster to not get 170 minutes over a whole league season. And obviously this would not apply to GKs. But atleast it would give a little semblance of power to the club who have sub-level players who refuse to leave.