#warning or #error would append lines to secondary outputs. I don't know if I have anything for #pragma to do.
Specific to GLSL #version checks if our parser supports the given version. Whilst #extension checks whether our parser supports a given extension possibly enabling or disabling it, or complaining about its support or lack thereof. Depending on what's requested.
Funny thing about discussing the C, GLSL, etc preprocessor on the fediverse. Directives get confused for hashtags!
4/4 Fin!