I remember toying with Polymer circa 2014, for some reason the word "transclusion" jumps into my mind, I remember being excited about it at the time. I barely remember what it means today though.
Polymer still haunts me to this day. It never made sense. It was literally designed to be deprecated. It's a big nasty polyfill for web components and it had/has a huge perf overhead. Not to mention it's ergonomics are just bad.
and some of the bad design decisions propagated to the web components implementation and Lit framework.
I believe "transclusion" was the Angular 1.x vernacular for "slots", but don't quote me on that ;-)