Hi, sorry for the delay, I've been away from home for a few days. The pull request looks good, thanks for putting in the work 👍

As for whether the flags are necessary: I know that /permissive- was necessary for sure and that it definitely could not compile without (I think there were issues linked to old versions of MSVC not correctly implementing two-phase name lookup). I don't know whether /Zc:preprocessor was ever useful though.

I plan to remove support for old compilers and to break some retrocompatibility in cpp-sort 2.0.0, on which I've been working for a few years, so for now I'm fine with being conservative and going the extra mile to let the 1.x.y branch target older compilers.