seeding mode in SQL Server 2019
Trace Flag 9567 : Enables compression of the data stream for Always On
Availability Groups during automatic seeding. Compression can
significantly reduce the transfer time during automatic seeding and
will increase the load on the processor. For more information,
see Automatically initialize Always On availability group and Tune
compression for availability group.

As the document says, just when automatic seeding process during. Synchronization doesn't relate to this trace flag.