I just learned about Pkl, so take this with a grain of salt. JSON Schema and Pkl seem to have some overlap. But JSON schema is not specifically designed for handling configuration and Pkl supports other formats like YAML.
I get where you're coming from, but JSON Schema still absolutely is the framework that supports YAML files and not the other way around. I've been using JSON Schema pretty heavily lately to write schemas using YAML, for validating YAML.
Possibly. My point is: despite having a common subset Pkl and JSON schema doesn't seem to be solving the same problems. But, I'm just learning about it, so I may just be wrong.