Many people were using the following code to exclude certain categories from appearing on their homepage

function exclude_category($query) {
if ( $query->is_home ) {
$query->set('cat', '-xx');
}
return $query;
}
add_filter('pre_get_posts', 'exclude_category');

A bug in a fairly recent WordPress update broke this (completely valid) code for everybody.

If a Chrome update breaks something, it's an annoyance, maybe. But if a WordPress update fails, at best it's a public embarrassment and at worst a disaster.

If automatic updates is a must, then make sure a lot of people try the update safely on actual production sites before pushing the update out to everyone else...