I feel like the state of the art is baked into the compaction logic, and I've had a lot of problems with compaction (absent other prompting) losing key bits of state. https://github.com/toon-format/toon is another interesting one, and I feel like it takes on a much more achievable goal - reduce whitespace and verbosity of JSON, not overall context compression.
Unfortunately, I saw some research paper suggesting that TOON doesn't help overall.
It's shorter than JSON, but novel/different enough that agents wasted more tokens thinking about it, interpreting it, and making extra calls.