Yaml or json is irrelevant for the files in .storage. The UI stores it’s config in text files which have a no published schema. It does this intentionally, because that way it can completely change the schema and organization of these files in any release (major, minor or patch). Doing so isn’t a breaking change because there’s no support for humans making and modifying these files. Hence no published schema or documentation on them.
Given that, why in the world would they make them yaml? That would be taking a significant performance hit for no reason. Parsing json is much faster then parsing yaml.