The modern version is tags, but they lack good sorting/viewing for tags
That is just dumbing yourself down in favor of machine classification or search algorithms while a simple solution exists for this problem for higher order classification and context, which tags alone fail to provide, even with sorting.
A tag with a single entry is useful. Chapters should never have a single entry.
Folders are great for when you don't know what you want and just want to browse or explore based on broad topics. Tags/indexes are better when you know basically what you want, and you just need a way to find it or items similar to it.
I don’t like being restricted to single folders, but I’d love to symlink projects into multiple folders.
This would work out well if they just added some graph functionality among tags.