Follow-up to #1824898: Convert drupal_weight_select_max variable to CMI system
The DRUPAL_WEIGHT_SELECT_MAX constant + variable was converted into a setting in system.site config object.
system.site is definitely not an appropriate home, and it makes no sense to have such a configuration value in the first place.
→ Convert system.site:weight_select_max into a #max_select_limit property on the form element itself.