This post seems to be suggesting that retention schedules backups, but schedule screen does that. Retention (any of the options) controls the retention, meaning what is retained rather than removed.
Smart and custom retention thin backups to increasing time interval between kept ones as they age.
This reduces storage use, however too long an interval may miss all copies of some short-lived files.
Duplicati.CommandLine.exe help retention-policy
--retention-policy (String): Reduce number of versions by deleting old
intermediate backups
Use this option to reduce the number of versions that are kept with
increasing version age by deleting most of the old backups. The expected
format is a comma separated list of colon separated time frame and
interval pairs. For example the value "7D:0s,3M:1D,10Y:2M" means "For 7
day keep all backups, for 3 months keep one backup every day, for 10
years one backup every 2nd month and delete every backup older than
this.". This option also supports using the specifier "U" to indicate an
unlimited time interval.
Above should also note the implied deletion after the longest time frame, which “U” can eliminate.
That was a late addition, and there’s lots of catching up to do in the help and manual (issue filed).
Volunteers to help with messages, manuals, code, and anything are scarce, and progress is slow.