Link bidirectionality

We got used to thinking about links as unidirectional.

The Web is a system where a page can declare a relation to other pages so that you can jump from one to another, but not necessarily both ways. Although we can address this issue by indexing pages and creating a graph that facilitates back-and-forth navigation, it is not a feature that people currently expect.

Before the Web, links were commonly perceived as bidirectional, where two entities were linked, and additional information was associated with that relationship. It is possible that Tim Berners-Lee (TBL) opted for unidirectional links as a compromise to enable permissionless page publishing or to maintain simplicity.

What could be the simplest mechanisms that can maintain bidirectionality while maintaining a permissionless environment like the Web?