Would be nice to backup the system without the dietpi_userdata for example and make use of some modification on the backup script.

Greetings cocoflan.

I agree, we merged this once to make it simpler, as well since in many cases system and software on /mnt/dietpi_userdata belong together (version/libs etc). However if there is much data stored, it can be annoying.

Currently you can exclude the data dir manually via dietpi-backup GUI.