The siprop=autopromote parameter of the Siteinfo API was generously added last year (T196059) for the very use case of XTools needing to programmatically determine when an autopromotion happens. We currently scrape InitialiseSettings.php (yuk!) as I was not aware at the time of implementation that a JSON format was available. After siprop-autopromote shipped, I had intended to update XTools shortly thereafter, but it got lost in my to-dos, hence I'm filing a task.

Note also removing reliance on noc.wikimedia.org helps us towards better 3rd party support (T344090) which has been in higher demand in recent years.