See https://nigeltao.github.io/blog/2021/json-with-commas-comments.html
for more information.
It is quite common to have trailing commas in JSON, and a lot of
parsers support it. I believe this could be trivially supported by
PHP, even if you skip adding support for comments.
Perhaps this could be disabled by default and made opt-in with a
option flag? json_decode already supports passing in options.
Is there any interest in this?