Earlier quoted context omitted.
I don't think this is a feature flag in the same way the rest of the discussion is using the phrase.
How so? C99+ support/conformance is a compiler feature. That feature breaks/conflicts with some aspects of C90 support, an existing, older feature. So you need a feature flag. Inside comiler there are various places where you have the equivalent of "if C90 do this, else do that".
"Feature flags" tend to be for behavior that is being developed and tested. The -ansi flag is more like configuration. It's pretty valuable to continue to support C90.