Marginally related: HyperScope by bootstrap.org (Doug Engelbart Institute). This is somewhat similar by providing hierarchical views into complex documents and referencing sections by ID, but is more realistic by doing this by a linear drilldown only.
Note regarding realism: In my naivety, I once expected option-based content organization and composition, much like in StretchText, to become an inevitable feature of multi-platform presentation, when mobile clients arrived. Surely, there was no way to present content in a manner that would suit needs both for mobile presentation and traditional desktop presentations at once (regarding length, level of detail and feature richness), posing a serious economic challenge for any content production. Well, then we got mobile first. ;-)
[1] https://hyperscope.org
[2] https://www.dougengelbart.org/content/view/154/86/
[3] https://github.com/BradNeuberg/hyperscope
Note regarding realism: In my naivety, I once expected option-based content organization and composition, much like in StretchText, to become an inevitable feature of multi-platform presentation, when mobile clients arrived. Surely, there was no way to present content in a manner that would suit needs both for mobile presentation and traditional desktop presentations at once (regarding length, level of detail and feature richness), posing a serious economic challenge for any content production. Well, then we got mobile first. ;-)