[1] They don't want to call it a fork, but when they invent totally new extensions to support things that already is solved by parts of the standard that they don't want to support, then it is a fork.
OpenAPI 3 was developed while JSON Schema progress was stalled due to the prior editors leaving and a new group of us (eventually) picking it up.
OpenAPI 3.1 will most likely include a keyword to allow using standard JSON Schema as an alternative to their customized syntax, and hopefully we can achieve full integration on OpenAPI 4. There are also some other ideas being explored for improved compatibility in 3.x.
Standards work is hard, but the relationship between the OpenAPI TSC and the JSON Schema editors is quite healthy and we are making good progress.
We've got some JSON Schema APIs in our services already and are using libs like the Python `json-schema` package to do validation with those. I also briefly experimented with Quicktype [2] to generate some TS types as a proof-of-concept.
We're getting ready to do a pretty big rework of a lot of our services, and I'm interested in any info folks can provide on pros and cons of using OpenAPI vs JSON Schema for API definitions, and tooling around request validation and TS interface / client generation.
[0] https://philsturgeon.uk/api/2018/04/13/openapi-and-json-sche...
[1] https://github.com/OAI/OpenAPI-Specification/issues/1532
[2] https://quicktype.io/typescript/