Backup of passive node of Microsoft Exchange Database Availability Group does not truncate transaction logs

Transaction logs do not get truncated after backup.
Microsoft Exchange VSS writer reports zero databases in its metadata on the passive node.
Microsoft Exchange Replication service VSS writer is disabled and it is not present in the list of VSS writers (vssadmin list writers) on both active and passive nodes.

Enable the Microsoft Exchange Replication service VSS writer. This VSS writer is enabled by default when you install DAG, however legacy backup software might need this writer to be disabled and that could be the reason why it was disabled manually.

Open Registry Editor and navigate to
HKEY_LOCAL_MACHINE\Software\Microsoft\ExchangeServer\v14\Replay\Parameters (Exchange 2010)
HKEY_LOCAL_MACHINE\Software\Microsoft\ExchangeServer\v15\Replay\Parameters (Exchange 2013)
Change DWORD value named EnableVSSWriter from 0 to 1.
Then restart the Microsoft Exchange Replication service.

Discovered by:  Jason Matthews