C++ does have its nice parts and it does have its uses but some parts truly are shit, get out of the Stockholm syndrome and admit it.
Let's get this straight: I'm not advocating for the preprocessor here. I will happily put it in the trashbin where it belongs as soon as I have support for modules in C++ (By the way, 90% of the code I write is written in the D programming language, which has modules, and no preprocessor).
I'm advocating for interface files.
Whatever mechanism you use, you still need a way to separate interface files from implementation files. Interface files can be header files, they can be java files containing a single java 'interface', they can be "D interface" files (.di). It doesn't matter which mechanism you use for this, as long as you have a way for your caller not to depend on the implementation of callee (which might, for example, directly depend on a specific audio API, for example).
My point is, if you want the caller to be properly isolated from the implementation of the callee, having to maintain signatures in an interface file is unavoidable.
Sure other languages 'manage' it fine. And if you want, you can put everything in your headers in C++, there's hardly any 'need' for .cpp files. Of course it will blow up your compile times, especially without precompiled headers (which are a pita with gcc, and relatively recent there too - gcc only started supporting them in 2006).
Don't get me wrong, I can forgive all shortsighted design choices and all the bullshit that piled up over the years. Nothing is perfect, true. But what C++ does to otherwise intelligent people.. This technological enslavement is just fascinating.