Indeed, the only Typescript feature I can think of that isn't purely for type checking or lifted from ECMAScript is Enums, and Enums have been around forever.
Well it has supports for JSX and decorators which AFAIK aren't in the spec yet. It also has a syntax for class mixins without actually doing any desugaring when the code is transpiled. So it's not quite just JS + types.
Ah, I forgot about those. Nonetheless JSX and Decorators were added to the language back in 2015, so they've been sticking strictly to ECMA features for nearly 3 years now.
It has public, private and protected members (checked at compile-time), which is a bit of a mess because ES is gaining an independent notion of private fields (checked at runtime).