I do strongly vote for that feature.

With incremental backups between fulls, we realistically only dare to set up weekly schedules.
The risk of a single corrupt incremental, rendering the whole backup void, is just to great for us when only performing a full once every month. (and don't mention yearly fulls...)
Also, the restore time becomes higher, with every incremental in the chain...I don't wanna imagine restoring a website from a backup with 30 increments.

Yes, differential backups will use a little bit more space, but in general it's no more than 5-10%
But if I can safely choose monthly fulls instead of weekly because of it, it will even save space and ressources.

Hey guys, any updates?

Incremental backups are not safe if one backup in the chain is broken the backup can not be restored. Differential backups are much safer and are a sweet spot between incremental and full backups. Please implement this feature!

I am using a pair of bash scripts I have created years ago in order to create a differential backup, keeping all history up to certain limits, and people continue to ask for it, even for Plesk installations.

Now that I have Plesk myself, I am facing the problem that backups are always complete and do not reflect changes in an easy way. With sites generating each backup several GB, this is a big problem. Also, when making cross server backups, they generate too much traffic.

A differential backup just has one good copy and the differences, and those are tolerable sizes even for huge sites.

I am willing to give you the scripts to look through and I eventually will make a version which works for Plesk based on the ideas we may have come up with.