I'm not sure. As far as I see, stacked PCB were always a thing. Sane designers would avoid it due to higher costs for connectors, assembling etcetc. Only if there is really a need to stack something (maybe hardware variation?). Isn't Eurocard the same concept, better connectors and more rigid?
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https://www.kosagi.com/w/index.php?title=Novena_PVT_Design_Source
https://kosagi.com/netv_hardware/
https://github.com/betrusted-io/betrusted-hardware