YAML (and TOML and even XML when it's being used in a tag-centric way instead of a text-centric way) are not markup languages. A markup language is one that allows you to assign meaningful annotations to regions within a chunk of textual content (e.g. "these three words are underlined", "this relates to footnote 3", etc.). YAML and TOML have both changed the meanings of their acronyms to try and distance themselves from their original misnamings as "markup languages".
Anyway, the GP's question is a good one. Where are the configuration/serialization languages that import the only good parts of YAML?
AFAIK, one can not subset YAML and get something coherent. But one can start from scratch and use its concepts. I imagine the languages are not there just because nobody got the time to do them.
