Currently the token SPT has no sink but I hope this changes in the future. When it was first created I remember talks of a potential sink being created being a hot topic. For the founders to not create an eventual sink for the SPT token that has the games name embedded in it would be (in my opinion) a massive blunder.
They seem to take care and create value for everything they have created within the Splinterlands game/universe so I have no reason to think SPT will be any different. But its entirely possible we never see a sink for SPT.