Since the update, it has not been possible to add new subcategories onto pre-existing, legacy categories.
I'm immensely grateful that the legacy subcategories can now be selected in the new Finder with the additional drop-down that appears if they are present.
However, if these legacy subcategories need adding to, there isn't a way to do this. In the Finder Editor "Add" is greyed out and the "Edit (Legacy)" is the only method of editing the legacy categories. Following this link though, takes you to Manage content, and entering a piece of content here in order to amend the categories informs you that this is all done in the finder editor now and new subcategories can't be added.
A way around this would be grand in order to organise pre-existing content further.
Hi Ben, Thanks so much for kindly sharing your feedback. Our advice would be to recreate the legacy categories that you continue to use them and subsequently manage them in Finder editor. Ultimately as a product we want to move to towards everything being managed in Finder editor but currently we need to support two different library systems since the legacy categories created in the old UI can have more levels of hierarchy than Finder editor can support, hence why the options regarding legacy categories are limited. Recreating the legacy categories in Finder editor, whether progressively or setting some time asides to do it will enable you to manage them in Finder editor going forward. I hope that helps.
The Access Product Team