Well it seems to work for TCP at least, which is where it comes from. Of course it's not the correct approach for everything, but calling it "one of the crappiest ideas in CS" might be a tad harsh.

EDIT: Of course there are better ways to be robust than to try to just accept whatever garbage is thrown your way because "be liberal in what you accept." So for example since this is about config files, you could easily just tell the user that their stuff is wrong and tell them how to fix it.

I can imagine several security issues with accepting any kind of input in your program.

Literally every language held as a shining example of postel's law is full to the brim with security issues.

The literal interpretation of Postel's Law has been considered highly detrimental for 20 years: https://datatracker.ietf.org/doc/html/rfc3117#section-4.5