Since gerrit 50140 was merged, MediaWiki has required the JSON PHP extension
to run. Unfortunately, I just found out the extension has a component with
a license that the Free Software Foundation considers to be non-free,
even though the extension is one of those bundled with PHP (and would
presumably be covered by the PHP License, which is a free license).
Until such time a version of the native JSON extension exists that does
not depend on any non-free component, MediaWiki should continue to provide
a pure PHP fallback under the terms of the GPL or another free license, or
at least make one easy to use if necessary.
Ideally, the fallback should implement the underlying json_encode and
json_decode functions, so MediaWiki extensions that call them directly
will work as intended.