Both Plotly.js and KnockoutJS are great examples of stability.
I think KnockoutJS may still the only (formerly) major framework that focussed only on doing data-binding really well without adding in loads of other functionality. As you say, a shame it died out.
Agreed on all points, there is an advantage in using whatever is currently flavour of the month though (more documentation, components, better integration with dev tools and such) but I used KnockoutJS for a couple of years and really liked it, it was a massive upgrade over jquery for a lot of UI stuff and brought some much needed sanity.
These days I use either React or Vue in the same role as the world keeps turning.
It's amazing they are still putting work into it for those people with massive codebases dependent on it though, anyone picking that 7-8 years ago definitely made a good bet.
I think KnockoutJS may still the only (formerly) major framework that focussed only on doing data-binding really well without adding in loads of other functionality. As you say, a shame it died out.
I don't know if KnockoutJS is dead, so much as done or perhaps complete. If all you want is data-binding it might be worth considering. It was released as recently as 5 months ago (https://www.npmjs.com/package/knockout) and it's still getting the occasional bug fix: https://github.com/knockout/knockout/pulls
I suppose it's not that trendy so good luck getting buy-in at a team level, but it would certainly still work for a bunch of different use cases.