That said, the use of empty alt tags is generally discouraged and not even allowed together with the ARIA presentation role in most cases.
This decision of theirs will steer developers towards non-compliant/asemantic code. It's really stupid.
https://html.spec.whatwg.org/multipage/images.html#alt
https://w3c.github.io/aria/#example-13
"In the following code sample, the containing img and is appropriately labeled by the caption paragraph. In this example the img element can be marked as presentation because the role and the text alternatives are provided by the containing element."
<img src="example.png" role="presentation" alt="">
So empty alt tags are not discouraged.
Empty strings are the way to go for decorative images