HI @verglor, welcome to the forum - and thanks for the suggestion!
There are a few similar discussions (such as Support pluggable filter addons to ease implementation of special filtering and [Suggestion] Filter “Exclude folder if it contains a file xy” just like .nomedia) but I believe the main reason there isn’t a .duplicatiignore type feature due to how Duplicati processes folders.
Currently the way Duplicati currently processes folder structures is that once a folder is ignored, nothing inside that folder is scanned. This is done for performance reasons, but a side effect is that an ignore filter at a higher folder might cause ignore filters at lower folders to never actually be seen. 
It can also start getting messy if people have multiple backup jobs looking at different content in the same sets of folders.