What I mean is that you (IIUC) can’t use an AGPL library in a GPL app without relicensing the whole thing to AGPL. For many larger projects relicensing is a huge hassle and often a non-starter if there aren’t very good reasons for it.

“just don’t enforce it” probably isn’t enough for most companies and projects

A good reason to pick GPL is if you want to allow GPL software to integrate yours and you don’t care that much about the AGPL clauses (e.g. because your app isn’t a server).

CC0 might be a good fit for trivial template repos where you don’t want to burden downstream projects with having to include copyright notices.

Projects for Apple platforms usually also use .h, where it could mean anything from C/C++ to Objective-C/C++.

In practice, Clang handles mixed C/C++/Obj-C codebases pretty well and determining the language for a header never really felt like an issue since the API would usually already imply it (declaring a C++ class and/or Obj-C class would require the corresponding language to consume it).

If a C++ header is intended to be consumed from C, adding the usual #ifdef __cplusplus extern "C" {... should alleviate the name mangling issues.