[__exit__]

What's the source of that document?

I found a page[0] with more info and link references to multiple sources that seems to explain everything in detail.

[0]: https://dev.to/aryanjnyc/ken-wheeler-and-dan-abramov-deactiv...

[sweeneyrod]

The last Tweet in the article you link

[k__]

Interesting that the Vue devs are the source of this.