I am not compiler specialist, but as I do not use GCC (it adds extra run-time environment (support) requirements when not in a GNU environment such as Linux - I am just wondering what effect this has (e.g., no deprecated message) when not using GCC.

Or, is this a "statement of direction" that only GCC (syntax) compilers are (going to be) supported?

(FYI: there are OSS projects that only accept GCC, and those tend to be non-portable (imho). The "solution" in those cases is to build an additional run-time environment. Personally, I consider that non-portable as I do not want the role of having to maintain security patches for a "non-native" runtime environment (i.e., the maintenance of the (native) rte is performed by the OS supplier, not by me).