What exactly? As mentioned, the UE_4_X_OR_LATER macros were introduced in the 4.17 release notes. The macro WITH_FORWARDED_MODULE_RULES_CTOR was introduced in the 4.16 release notes, as part of the change from TargetInfo to ReadOnlyTargetRules. I didn’t learn about these from the release notes themselves (I don’t remember where I saw them first, but I think probably in some question on this site), but that is the best reference I could find for them (besides the source code).