Nit: these are not informal rules, they’re clear UBs, they’re “just” not checked. An informal rule would be “don’t use gets” before it was deprecated, or “don’

Nit: these are not informal rules, they’re clear UBs, they’re “just” not checked. An informal rule would be “don’t use gets” before it was deprecated, or “don’t use the str* functions”, or “don’t pass a user-provided format to printf”.