In the same way that we add a description for a card in the New UX there should be a way to add a description per line item. The users should be able to hover over the line item and information will pop up.
As of now we don’t have this feature in anaplan to add description per line items that can be published in NUX.
A similar feature in the NUX could be Comments or History for Page Editors. In the comments or history view the Page Editors can maintain the notes about the context for individual pages. This could be useful as some pages were used in one way for a particular group/role in classic dashboard, but the page is now used for a broader audience or for a completely different audience.
Unfortunately, it's not available so far.
I think you should publish this great idea to an Idea Exchange platform. It will be reviewed there and with anough votes it can be added in the future releases.