This is the heart of the problem

Yes, I definitely know that. But my point is that the existence of these highly optimized versions of strcpy() doesn't magically get you a highly optimized version of strlcpy(). There are plenty of functions in glibc which have not been optimized in this way, and there is no particular reason to believe that strlcpy() would magically get this support just by being included.

Someone needs to do that work, and even if the glibc developers could be convinced to add strlcpy() etc. to the library that's no guarantee they'd be motivated to generate highly-optimized versions.

And if someone WAS motivated to generate highly-optimized versions, then those could be used external to glibc just as easily (in fact, possibly more easily since you wouldn't have to worry about which glibc version you were using).