At least for now, Query Store can only record query performance in databases that are read-write. Once you go read-only you can review the performance of past queries, but you can’t track the performance of anyone who queried the database after the point it went read-only.
At least for now. Query Store is such an awesome feature that perhaps this will change in the future. (I don’t have any inside info, only optimism.)
That’s a little bit of a letdown, but makes perfect sense.