The CSS @apply rule no longer has a “champion” (in ES parlance), so it has essentially reverted to Stage 0 (though it is still behind a flag in Chrome), and it is on a path to extinction.
The champion of the feature was previously Tab Atkins, who has since moved on to a better solution for the shadowdom [citation, citation]. Unfortunately, this leaves regular "lightdom" elements without a possible feature to store a set of properties in a named variable, and then reference them in other style rules, reducing the amount of effort required to keep your stylesheet consistent as things change in the page. [citation].
If anyone wishes this feature to survive, it needs a new advocate within the CSSWG.