(I work at Notion)

Notion has transclusions. In the product we call them “synced blocks” (https://www.notion.com/help/synced-blocks), but the implementation type name is transclusion_container and transclusion_reference.

We also have backlinks. These used to be to prominently feature featured but these days they’re still supported but collapsed by default when viewing a page.

It would be cool to build Xanadu style multi-pane view but users tend to be a bit confused by horizontal scrolling content containers.

The CrossLine outliner (https://github.com/rochus-keller/crossline/) has transclusion and backlinks as well. Very useful concept indeed.

I don't use Notion heavily, but I've worked with any number of local editors that support transclusions and backlines. I think this is a wonderful system for individual and organization use. Basically, when all the docs are owned by the same entity, and there's no microtransaction BS, or the misguided idea of perfect traceability of every word on the screen (as opposed to just, you know, copy and paste where appropriate), it's blissful.

But extending that to the whole world's publishing needs? Hah, yeah, no way. I think that these systems, and especially the web, are more vast than Nelson's ideas could possibly have scaled to.