…and I’ll add that interpreting the zone serial number is particularly, well, not smart. The only actual requirements for the content of this field are in RFC 1982. Apart from that, it’s an opaque value, i.e. not to be interpreted. The recommendations that lead to the warning are operational in nature, to avoid common pitfalls when managing DNS serial numbers, but they are still not meant to encode information in the serial number that anyone could rely upon. These warnings can be informative to a novice DNS server operator. Nobody else has any business taking these warnings as a sign of bad practice.