First posted on /r/javascript, but I think it's worth posting here too:

I was a member of the YUI team until a few months ago. I'm still at Yahoo now, just on a different team, but just wanted to give my own thoughts on this (I don't represent the company or the YUI team).

My software engineering career started with the YUI team - I actually joined as an intern at Yahoo because of a Reddit post on /r/javascript. I was pretty new to engineering in general back then, and as a biology major with no real professional experience, I didn't have an easy time getting internships. Jenny, the manager of the YUI team back then, really took a chance on me, and that really changed my entire career path.
I solved a bunch of YUI bugs, added a few features here or there, and I always tried to help other folks on #yui on IRC, the mailing list, or in-person here at Yahoo, which I really enjoyed. I learned a crazy amount of JavaScript, some pretty advanced debugging / performance profiling techniques, and even gave some talks. Eventually, a lot of people always came to me first whenever they had a question about YUI, which was pretty cool.

From the view of some people in the JavaScript community, YUI was always considered a huge, monolithic framework that was only good for widgets. I never thought that was the case - YUI pioneered a lot of the techniques that are popular in advanced JavaScript development today, like modules, dynamic loading, and creating logical view separation in your code. A lot of the influence in RequireJS / CommonJS / ES6 modules can be seen from what YUI did first, which people used to consider "over-engineering".

With a lot of new development in JavaScript though (data-binding, tooling like Grunt / Yeoman, promises and other async handling techniques), it was always hard for YUI to keep up with new features while still being able to maintain backwards compatibility with the constantly deploying products that people were building at Yahoo. We had to support product teams while also building out the framework at the same time, and making sure the user-facing products were the best was more important. Eventually, it was hard when developers who were familiar with newer JavaScript tools tried to use YUI, but ended up having to spend quite some time with the framework just to get it working with the rest of the JS ecosystem.

In the end, I wasn't involved with this decision, but I think it was the right thing to do. A lot of the YUI (now YPT) team and other front-end teams at Yahoo are now working on helping out with more cutting-edge core JavaScript work, like internationalization (https://github.com/yahoo/intl-messageformat) and ES6 modules, as well as building out components for newer frameworks like React and Ember (https://github.com/yahoo/flux-examples). Yahoo still has a lot of really strong front-end developers, and working on these more important core components is more beneficial to both Yahoo and the JS community as a whole, than continuing to maintain a framework that's a walled garden.

The one thing to take away from this is that no technology lasts forever, and in the end, what the user sees is the most important, whether it's JavaScript, Android / iOS, or holographic smartwatches.

I'll be a bit melancholy today, but I'll raise a glass to YUI tonight. RIP.