Edited title to not be misleading.

array_key_exists() is NOT deprecated, the deprecation is using the function with objects (i.e. something like Joomla\Registry\Registry which implements ArrayAccess meaning you can work with the registry object as an array).

$thing = new Joomla\Registry\Registry;
$thing['foo'] = true;
$thing['bar'] = true;

array_key_exists('foo', $thing); // emits deprecation because an object is being checked

$otherThing = array();
$otherThing['foo'] = true;
$otherThing['bar'] = true;

array_key_exists('foo', $otherThing); // this is OK

There are instances of using the function with objects

It's possible. Could you point to some instances?

Closing for now - will keep an eye on my error logs

If you compare two versions of an article you will get the error
Deprecated: array_key_exists(): Using array_key_exists() on objects is deprecated. Use isset() or property_exists() instead in \joomla-cms\administrator\components\com_contenthistory\Model\CompareModel.php on line 104

please test #27493
closing this issue in the hope that all 7.4 compatibilty isuues will have a proper issue

Please test #29430 as it fixes the same issue on J3 as well.

I have just changed

return array_key_exists($index, $this);
to

return array_key_exists($index, (array) $this);
It is working. I just type cast in Array datatype