Does anyone remember a paper about a low level data format that was a kind of structured JSON thing, but it was a binary format and gracefully simplified into a raw string or raw int if that was the only data. It was definitely posted here at some point, maybe 5yrs ago though…!

https://protobuf.dev/ ?

I wasn't here, but maybe cap n' proto? https://capnproto.org/

CBOR? https://cbor.io

This isn't what you are asking for, but the question and responses fired these neurons in my head: https://altscript.com. I see Altscript as kind of like JSON++ or Rebol++ or Lisp++. FYI, YMMV.

Infra https://sites.cs.ucsb.edu/~holl/pubs/Hall-2017-Onward.pdf

Yes it was Infra! Absolutely impossible to Google name. Thank you! Website has been down for a while it seems but Wayback has it http://web.archive.org/web/20210304115826/http://infra-structure.org/