The reserved prefix $ answers the question: "how can we let people name, structure, and nest their tokens however they want, while future proofing the spec?". More on that rationale: https://tr.designtokens.org/format/#character-restrictions

As for the "pretty weird" aspect — it's definitely uncommon but it's been seen before: the $ prefix is also used by the JSON Reference ($ref): https://www.ietf.org/archive/id/draft-pbryan-zyp-json-ref-01... — and just in case, we ran this syntax past Ben Hutton (inventor of JSON Schema) to ensure we weren't doing anything silly.

You can usually do this by only being additive with the spec, and by never mixing user-defined keys and spec-defined keys in the same object.

I've only ever seen $ used before with meta properties that work across all schemas like the $ref you mentioned and $schema.

There are multiple ways to solve this, we've explored a few and we found that clearly differentiating user inputs from the spec keys was the right way to go for this use-case and our audience.

That said, there definitely are other valid ways to solve it.

Not just weird, it kinda overlaps with json schema builtins like $schema, $ref, $id, etc