They did not just broaden the API. Previously, extensions were written in Javascript, roughly following open web standards[0]. This let you write your extension (nearly) once and have it work in all web browsers.

Now, all of those Javascript extensions are deprecated (users can no longer install them). If you want your extension to work, you now have to rewrite the majority of it from scratch in Swift[1]

[0] https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/Web... [1] https://developer.apple.com/documentation/safariservices/saf...