This is very interesting to me.

Is the reasoning behind this documented anywhere?

I could only find instructions the coding guideline indicating as much (here: https://github.com/Microsoft/TypeScript/wiki/Coding-guidelin...) but couldn't find why. I'd love to know why, may better inform my usage of typescript.

For better or worse? I've avoided classes in our codebase because I don't like the lack of privates via closure (revealing module pattern) and it's transpiler overhead, but it's a very powerful organization tool. I've found code to be much neater in the class format.

Looks like C++ circa 2008. Careful with those abstractions. That seems a stone's throw away from template metaprogramming.