In tests that I have done, not all time zones are returned by this function. For example the following aliases Asia/Katmandu and Asia/Calcutta are not returned, but these time zones are supported in tests that I have done such as the following:
<?php
echo date_default_timezone_set('Asia/Calcutta');
?>
result:
Fri, 19 Jun 2020 03:26:52 +0530
UTC+05:30 is the correct time zone for Calcutta.
Asia/Katmandu and Asia/Calcutta are aliases for Asia/Kathmandu and Asia/Kolkata respectively (which can be found in the list)
Hopefully this helps some people because without these time zones appearing in the list one might not know that they are supported or even exist.