Correction: in the original second paragraph, the fourth sentence obviously has the case backwards.  It should read:

"When using a *Command Line Tools installation (no Xcode)* in previous releases of macOS, I believe it was the case that essentially the system root directory ('/') was searched by the compiler tool chain for header files, for libraries, and for frameworks, and for frameworks the long standard fallback framework search path order was honored by default: first search /Library/Frameworks, then /System/Library/Frameworks. [...]"