Hi,
Modifying the sidebar to add a new filter would require doing some changes to the code base -- particularly to PHP and especially ElasticSearch.
How familiar are you with the AtoM code base? There isn't any documentation available on making those changes, but you if you are well versed in the code base there are some tips we can provide.
As far as we know, no one has customised the sidebar. However, if there are any other users out there with some custom code, please let us know!
Best wishes,
Sarah
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Sarah MasonContributor Success Specialist
Artefactual Systems Inc.
she/her