As a note, incorporating Vue 3 also removes grade A compatibility for Safari < 10 (Safari 9.1) and Android 4.X lines, for the same reasons that IE 11 is not supported (lack of support for Proxy, if I am reading MDN correctly). Compatibility#Special treatment for IE11 could perhaps be adjusted, or perhaps both should be a note for Template:Compatibility browser instead of a section-proper. (Or perhaps its own row, "Browsers without Vue 3".)
Topic on Talk:Compatibility
The only reason I bumped into this is that some of us on Discord were throwing around sending VisualEditor to clients as WebAssembly, which has very similar Modern requirements according to CanIUse: it would require WMF to drop Modern support for Safari 9, Safari 10, and Android 4.X, which are almost the same requirements for Vue 3.
The eventual dropping of IE11 is essentially a more explainable version of the technical requirement for ES6, likely a year from now. See T178356#6632565 and startup.js isES6Supported for details on what that exactly would entail. There hasn't been a decision yet and the exact definition of "ES6 in practice" is still relativaly vague since some parts of the ES6 specification are still not implemented today even in the latest versions of Firefox and Chrome, despite ES6 (aka ES-2015) now being five years old and ES-2020 spec features having already been released in most browsers.
Having said that, it looks like Safari 9-10 and Android 4 indeed might end up on the side of Grade C, depending on what subset of ES6 we are going to check for and require.
Raising Safari Grade A support to 10.1/10.3 and dropping Android 4.x would also have the nice side effect of making CSS Grid Layout permissible to use without too much fiddling with fallbacks. Given the numbers are so low (<0.1% and 0.22%) it might even be worthwhile upping the requirement independently of ES6 and Vue 3.
@Xover That's right, we tackle these separately in “Basic” browser support. CSS standards are mostly aimed at in basic support (I just stumbled upon this discussion). See for example https://phabricator.wikimedia.org/T290815