TL;DR
You can now @import other files containing Include/Exclude Patterns into the filters file by using
@/the/full/path/to/the/customised-filters-file
@/the/full/path/to/the/some-other-filters-file
@/the/relative/path/to/third-filters-file
other filters below
Detailed explanation
Now that Filter extension by a-s-z-home · Pull Request #514 · gilbertchen/duplicacy · GitHub created by @a-s-z-home is merged, 
users will be able to use multiple filters files much more easily by just @ importing them into the default .duplicacy/filters file!
This is helpful as now one can finally backup its own custom filters template by storing it outside of the .duplicacy/ folder, and just linking (importing) it.
This is also helpful as you may split gigantic filters into multiple smaller ones and just @import them one by one.
I think an example is due:
Let’s say you’re using my filters template located in the following folder tree:
some/folder/path/
duplicacy-repository/
.duplicacy/
filters <- this is the normal filters file
duplicacy-utils/ <- this is my utils with the filters file
filters/
filters_general_usage <- this is my filters template
But you want to modify some of the filters in my template with a filters file which is located in
another/folder/path/
my-customised-filters-file
All you have to do now is to paste these 2 lines the .duplicacy\filters file:
@another/folder/path/my-customised-filters-file
@some/folder/path/duplicacy-repository/.duplicacy/duplicacy-utils/filters/filters_general_usage
It is important that your file is the first, and my general file is the second (or the last if you have multiple filters files yourself).
Now you can easily upgrade my filters template whenever i do it and by adding to the repository your own filters file you’re making sure you never lose it!