Without knowing the answer or what the query store is, this question is guessable with a solid understanding of the system databases. But, it got me to thinking. It makes sense for this feature to not be available for the tempdb. It makes sense to have the ability to enable it in the model database to provide the ability to have it enabled for all newly created databases. The quandary is what will happen if you enable on the model database, then stop SQL and delete the tempdb files. When SQL creates the tempdb from the model database, will it succeed, or will it fail and stop SQL?