Nope, was marked as WONTFIX: https://bugs.webkit.org/show_bug.cgi?id=182671

There was some more discussion in the webkit standards but looks like the answer is still unchanged: https://github.com/WebKit/standards-positions/issues/97

for over 10 years now Apple has stated it won't implement Customized Built In Elements ( "is" extends )

If you want to dive in deep: https://stackoverflow.com/search?q=liskov

Here is Apple debating all non Liskov believers: https://lists.w3.org/Archives/Public/public-webapps/2013OctD...

*going back over 10 years to 2013, Who said Web Components was a fad*

Probably not. But the global attribute "is" is not necessary. It is just a bit more obvious.