Does use of C++17 std::filesystem REQUIRE MacOS 10.15? (Xcode 11.1)
It means that (using Apple's tools/libraries), you cannot use std::filesystem in a project targeting a version of Mac OS before 10.15.
If you have a new enough Xcode, etc. you should be able to build a project on 10.14 (say) that targets 10.15 and uses filesystem types and calls. But it won't run on 10.14.