I wouldn't call it recent. Boost has been doing it since it was started in the late 1990s.

Boost or C++ stdlib headers are different than STB-style single-file-libraries though in that the implementation code is inline and needs to be parsed each time the header is included.

Boost did it because C++ template definitions must be in headers.

That seems unrelated to this idea that C code using it is equally reasonable. That's a developer choice.

The original C++ templates used to be entirely contained in the headers. I assume that is no longer the case, but it has been a long time, since I wrote C++.

When I was writing template SDKs, the headers were pretty much the entirety of the library.