Avoiding designing in DDoS relay/amplication vectors requires luck or intention, not just making the protocol small.

Every reasonable connectivity provider would pay attention to this info, or face intense complaints from its users with shared and dynamic IPs. It would identify sources of attacks, and block them at higher granularity level, reporting that the range has been cleared. (If a provider lied, everyone would stop believing it, and the disgruntled customers would leave it.)

For shared hosting providers it would mean blocking specific user accounts using a firewall, notifying users, and maybe even selling cleanup services.

For home internet users, it also would mean blocking specific users, contacting them, helping them identify the infected machine at home.

It would massively drive patching of old router firmware which is often cracked and infected. Same for IoT stuff, infected PCs, malicious apps on phones, etc. There would be an incentive to stay clean.

If your computer is infected, I don't want to talk to you for a month. If it continues to be infected, I might up that to a year, or permanently ban you.

It's your problem. Go fix it.