Did the other feature have anything to do with modules? If so, I can't really blame TypeScript too much for that, since the JavaScript ecosystem is pretty fragmented there and TypeScript has to support all the different things that are in use with a reasonable interop story. Build tools that work purely with JavaScript source code also have to deal with this problem.
Otherwise I'm not aware of any cases besides decorators where TypeScript did something that was incompatible with a later ECMAScript development.
I really don’t remember, sorry. I just scanned through https://github.com/Microsoft/TypeScript/wiki/Breaking-Change... and nothing jogged my memory; I’m wondering if it actually was something to do with decorators, though I still think probably not. I investigated a bit at the time because I was looking into using TypeScript on a certain project and the changes would probably affect me, but I wasn’t actually using TypeScript at the time, and it didn’t stick in my memory. Something about some change being spread over a few versions with deprecation followed by a silent breaking change in the generated code, but I can’t remember what.
Otherwise I'm not aware of any cases besides decorators where TypeScript did something that was incompatible with a later ECMAScript development.