IIRC, Angular already ships with its own similar polyfill, so I'd guess it tries to fallback to native if possible. React doesn't really concern itself with styles anyway, though I guess they might need/want to make sure they're not standing in the way of using the shadow DOM.

Of the less-major ones, I only know that Polymer uses it - which makes sense, since this is one of the APIs it was explicitly created to push.

Not a major framework, but UIBuilder lets you use Web Components using JSX, just like React:

https://github.com/wisercoder/uibuilder

Polymer is pretty integrated with shadow dom iirc, although last time I used it I had to use a bunch of polyfills.

You don't need any polyfills on Chrome, Safari, Opera and soon Firefox obviously. Edge is the only holdout.

The way I see Shadow DOM is that it's an environment for an encapsulated web component or embedded app to run within. Most frameworks can be mounted to the DOM at a single spot, so should be pretty compatible, I would think.

There are also a lot of existing web components libraries that have been developed in anticipation of first class support for Shadow DOM in all browsers. See https://en.m.wikipedia.org/wiki/Web_Components#Libraries

Perhaps not as 'major', but Aurelia is intentionally aligned with web components and other standards:

https://aurelia.io/

Is it still a thing?

For custom elements, here's a list at https://custom-elements-everywhere.com/

Angular supports this for quite some time.

It does, but emulated by default, you have to explicitly add ViewEncapsulation.NATIVE to your component decorator, or NONE if you wish/are forced to disable it.