The native toolchain on my embedded board is a mess. You don't want to learn to deal with it if you don't have to. This comment applies to every embedded toolchain I've ever worked with. Even doing embedded linux with yocto is a mess of a toolchain and it is the best attempt I've ever seen at creating a good embedded toolchain. The problem is just messy.

Usually that boils down to "we don't want to bother with the vendors IDE", e.g. MPLAB.

I know C and C++ since 1992, used them across multiple OSes, hardly seen anything that would motivate the fashion of header only files.

You've never in 28 years lost a single day to a compiler/linker flags/version mismatch and some random tool? I find that difficult to imagine. The horrifying alternative would be that such situations had been encountered but considered justifiable productive work

No, because I either used what was provided by the OS vendor, or library providers that were selling libraries for the deployment we wanted to do.