Available from version 1.39.0 (Gerrit change 822733)
Use this hook to add additional validations for JSON content pages.
Define function:
public static function onJsonValidateSave( JsonContent $content, PageIdentity $pageIdentity, StatusValue $status ) { ... }
Attach hook: In extension.json:
	"Hooks": {
		"JsonValidateSave": "MediaWiki\\Extension\\MyExtension\\Hooks::onJsonValidateSave"
Called from: File(s): content/JsonContentHandler.php
Function(s): validateSave
Interface: JsonValidateSaveHook.php

For more information about attaching hooks, see Manual:Hooks .
For examples of extensions using this hook, see Category:JsonValidateSave extensions.

Details edit

This hook is only called if JSON syntax validity and other contentmodel-specific validations are passing.