Yes, this is one option. You’d have to use the --purge command to remove the data from the back end.
This will also reduce the storage on the back end. The setting to use depends on your needs.
I personally like having a multi-tiered retention. I set mine to “custom” and use “7D:0s,3M:1D,1Y:1W,99Y:1M”. This tells Duplicati to keep all backups from the past 7 days; beyond that keep 1 daily backup for 3 months; beyond that keep 1 weekly backup for 1 year; and finally beyond that keep a 1 monthly backup for 99 years.
(Also note that you may run into a bug enabling retention if you have a very large number of backup “versions” … the latest canary version has a fix for it.)
The amount of data freed up on the back end by adjusting retention depends on several factors.
That’s basically the same as changing the retention setting. If you mean temporarily, you could change the retention, run one backup (so that Duplicati has a chance to apply the retention settings once and delete older versions), and then change retention back to “keep all versions” if you want. Of course if you set it back to keep all you may run into this problem again in the future.
Yep that’s always an option too…