How to responsibly discontinue receiving email at an old Internet domain?

As is noted already in the question, there is a conflict between the desire of not accepting incoming email and the idea of being able to contact the domain owner via email by convention-based email addresses.

There is just no way around those two goals being in direct conflict, you will have to decide if you are going to accept email or not.

If you decide that you are not going to use this domain for email at all, I think this would be a good citizen approach:

domain.example. IN SOA name.of.nameserver.example. hostmaster.primarydomain.example. 2019040100 3600 1800 3600000 7200 
domain.example. IN MX 0 .
domain.example. IN TXT "v=spf1 -all"

• A NULL MX to indicate that you will not accept delivery of mail to this domain (avoids delivery attempts based on address records (A/AAAA)).
• An SPF policy that indicates that no one should expect to receive any mail from this domain (makes the domain more hostile to spammers).
• Ensuring that the SOA RNAME (second value in SOA) is actually a valid email address at domain that still accepts email (eg hostmaster.primarydomain.example. if [email protected] is a good address).
  (No suggested changes for the other SOA values, they are only present in order to show a valid record in my example.)