TypeScript already is very stable and mature.
I'm using it for quite some time and I didn't have any problem. Each new release it gets better.
And you don't have to take my experience with it as a good sign of its maturity.
Take in consideration that developers from angular and reactjs are using it as well as working closely with TypeScript developers to improve it even more.
Also, the beauty about TypeScript is that if you don't want to use it you don't need to.
There are 2 ways cocos developers can support TypeScript:
1 - they develop in TypeScript language.
In this way, the result for those that don't want to use TypeScript
Also, I know many people have prejudice to Microsoft but it's really a great product. I used to have it too.
I'd like to invite you to watch this video and , in case you liked what you watched, give TypeScript a try, putting any negative feelings apart while testing it.
I'm pretty sure you like it too.
PS: I'd like to invite cocos developers to watch the video above to know a bit better about TypeScript. I'm pretty sure you will love it.