Upgrading SQL 2016 AGs to SQL 2019 question
&quot;Synchronized/In Recovery&quot; means the database is synchronized with the primary and applying log records continuously from the primary.  This keeps the database up-to-date against the source 2016 database.  The database cannot be used while in this state, not even for read-only queries.
If you were to failover to a database in that state, the server would run recovery on the database once it has applied all the outstanding log records in the redo-queue.  At that point the database comes online, and can be used by clients.  However, once recovery runs, and the database engine upgrades the database to SQL Server 2019, it cannot be restored onto prior SQL Server versions - i.e. you couldn't fallback to a 2016 server.
Once you complete the upgrade of the last node to SQL Server 2019, database status should return to normal on the readonly secondaries.