I once did a “Hierarchical project” test in KiCad V5.1, and back then it did seem to work.
I had a “global project”, and some “sub projects”. The Idea was a (fictive) project which had a main PCB and several daughter boards. In the “Global project” you had an overview of the whole schematic. You could use the hierarchy to switch between the different “daughter boards”, and you could do ERC checks on the whole project.
In each of the sub projects, only a single PCB was visible, and this worked for designing each PCB in the project.
When I did a search to find my thread above, I also found quite a lot of interest from others in this issue:
https://forum.kicad.info/search?q=multi%20pcb%20project
My best guess is that “The KiCad Developers” are both aware and interested in this, but there is no consensus or time table of how and when to support this and how the pieces should best fit together. One PCB per project works quite well, and there are lots of more pressing issues to improve for KiCad and implementing real support for this is probably several years into the future.