Configurable product should display all simple products, also the simple products which are out of stock.
For example an item has the sizes S, M, L, XL. And M is out of stock.
At this moment it only shows S L XL.
I need Magento to show also the size M (with an "out of stock" css class added to it).
How can I fix this for Magento 2.3(!) ?