Xcode 15 warns you when targets contain a Build Carbon Resources phase. The solution is to drag all your .r files to the Copy Bundle Resources phase instead, and deleting the now-empty Build Carbon Resources phase. Beware that after this change your project is unlikely to build correctly with Xcode 14.
While both versions of Xcode support .r files in the Copy Bundle Resources phase, invoking Rez to compile your inputs, Xcode 14 incorrectly outputs a .rsrc file whose name matches the input .r file (MyResources.r → MyResources.rsrc). This is wrong, since the Carbon Resource Manager recognizes only one file containing Carbon resources (AFAIK) and its name must match the bundle executable. If your executable is named Foo, your Carbon resources should be in a Foo.rsrc file or else they will be ignored.
It is unclear if Xcode 15 is smart enough to merge the output of multiple .r files into a single .rsrc. Lucky me I don't need that functionality for my targets but it's something else to look out for.
...and for those in utter disbelief that in 2023 there are still people talking about and using Carbon Resources as designed and implemented in the 80s... welcome to the world of third-party plug-ins for Adobe apps ;-)