I haven't done any exact test, but it looks identical to how the json_object_agg function in postgres works, but with a different syntax.
Is it really that different to not even warrant a note in the table?
Clearly you know better than I do, but the big red X surprised me as the fantastic JSON support is one of the reasons why I tend to reach for Postgres so often over other DB engines.
A big red X doesn't mean the database doesn't have other, similar but proprietary functionality. It just means: it doesn't support this function in a standard confirming way yet.
There is nothing against using non-standard functionality in that case. In this particular case. PostgreSQL got JSON support (~2012?) long before it was added to the standard (2016). Obviously, their JSON functions differ from the standard (as they didn't lobby their functions into the standard).
However, slowly but surely database will add the standard functionality, which makes life easier.
Having that said, the standard JSON_OBJECTAGG is more powerful as it gives you control over null handling (NULL ON NULL -vs- ABSENT ON NULL) and allows checking for duplicate keys ([(WITH|WITHOUT) UNIQUE KEYS]).
Maybe add some little footnotes to the X? It took me a few minutes of thinking "Of course postgres has json object aggregation here it's working perfectly and it's documented" before I realised that sure it existed but it's not the standard version, at all, which is probably why it's marked as a no, because it's not Troel's page and Markus really really cares about the standard.
https://www.postgresql.org/docs/current/static/functions-agg...