Snapshot Files deleted when "@immediate_sync = N'true'" and "@retention = 0".

As mentioned in previous post the @immediate_sync = N'true' allows you to add a new Transactional Replication subscriber anytime during the Retention Period as both the Snapshot files on disk and the “cached” transactions stored in the distribution database will be available to bring a subscriber up-to-date with the Publisher.

One small catch:

SQL Distribution Cleanup job automatically purges the Snapshot folder of all snapshot files older than 72 hours (default). This occurs even when "@immediate_sync = N'true'" and "@retention = 0".  If you need to keep the Snapshot files longer, increase the @max_distretention in the "Distribution clean up: distribution" cleanup job.

EXEC dbo.sp_MSdistribution_cleanup @min_distretention = 0, @max_distretention = 72

If you increase the max_distretention cleanup value larger then Retention Period, both the Snapshot and the cached transactions will be available for a new subscriber.  However, setting @immediate_sync = N'true' can also contribute to Distribution database growth.