They have blown things out of proportion (see parent comment). Only extremely minor browsers will be affected. Chromium (Chrome, Edge, Opera), Quantum (FF) will be fine. I suspect even the likes of KHTML will add support for it.

Pale Moon is a fork of Firefox made before the removal of XUL (as I believe is their selling point). They would have to back-port Web Components (which may be impossible due to Quantum) or write it from scratch. This is all really hard for them because they likely have fewer than 10 active developers.