It's here: https://trello.com/c/V2zvGqou/21-share-an-item-rather-than-a-document
And agreed, that's the only truly convincing argument we've heard that document & folder structure just can't do. Although I still tend to think people propose this because of the existence of WorkFlowy. For example, you don't see people suggesting that Google Docs should allow sharing a section or a paragraph (maybe some do, just not as often brought up as with Dynalist).
Anyway the feature request is up there in the roadmap, and you're welcome to vote for it if you find it useful.