The Query Store feature is a bit unique in that its status can change without user interference, which means it is important to understand how changes to Query Store are logged. A couple weeks ago John Deardurff posted a question on Twitter asking specifically whether the SQL Server ERRORLOG is written to when the OPERATION_MODE changes to READ_ONLY because MAX_STORAGE_SIZE_MB is exceeded. I had never tested to confirm, but I know there is an event in Extended Events that will fire when the limit is reached. I also know that when a user makes a change to a Query Store setting, it is logged in the ERRORLOG.
Click through to see how to watch for this and what the changes look like.