I just checked YAML 1.2 and it seems that 1024 limit length on keys still in spec (https://yaml.org/spec/1.2.2/, ctrl+f, 1024). So any JSON with long keys is not compatible with YAML.
Another reason to have a limit well below the computer's memory capacity is that one could find ill-formed documents in the wild, e.g., an unclosed quotation mark, causing the "rest' of a potentially large file to be read as a key, which can quickly snowball (imagine if you need to store the keys in a database, in a log, if your algorithms need to copy the keys, etc.)
I assume JSON implementations have a some limit on the key size (or on the whole document which limits the key size), hopefully far below the available memory.
I assume and hope that they do not, if there is no rule stating that they are invalid. There are valid reasons for JSON to massive keys. A simple one: depending on the programming language and libraries used, an unordered array ["a","b","c"] might be better mapped as a dictionary {"a":1,"b":1,"c":1}. Now all of your keys are semantically values, and any limit imposed on keys only makes sense if the same limit is also imposed on values.
Yes absolutely, in practice the limit seems to be on the document size rather than on keys specifically. That said it still sets a limit on the key size (to something a bit less that the max full size), and some JSON documents valid for a given JSON implentation might not be parsable by others, in which case the Yaml parsers are no exceptions ;)
I'm not even sure why I'm playing the devil's advocate, I hate Yaml actually :D
