You can still run JS or WebAssembly based XSLT transformer
The problem is that XSLT has so few users that the browser writers don't want to waste any more time on it. Those few people who wrote their site around it will have to either rewrite it to use something else or add some third party webassembly implementation themselves.
So an XML document would be parsed and processed by the XML engine, trigger XSLT processor, which would then trigger again the XML processor again and the HTML processor.
So XSLT processing for XML/XHTML is deeply embedded. Dropping it removes a lot of corner cases that AI is good at exploiting.