In 2021 I got back into it:
I took a look at Blazor. Ok, that's just web components, where the file name is the component name.
I took a look at Svelte. Ok, that just web components with reactivity. Svelte looks like the inspiration for Blazor.
I took a look at Angular. They have components now or did they always have components?
I took a look at React. Look, they're starting to use Polymer type components inside React components.
I took a look at web components, not longer called Polymer it seems, and wow, they've made a lot of progress. Salesforce.com is a big user (a company I respect for their tech) and Google itself is a big user. Google says Web Components are everywhere and I'm using them a lot without knowing it.
So have Shadow DOM components arrived?
https://developer.mozilla.org/en-US/docs/Web/Web_Components
It is NOT about Technology, it is about Who creates Technology.