Although there is not that single solution for all occurancies, most of the systems are able to handle custom fields. This should allow to add such a field to DAVx5 too. I think it would always be better to cover at least 70-80% of the systems instead of 0 %. I just had a look on it and checked how other systems are handling a death day field. Here are my findings:
Nextcloud 15.0.2/Contacts 3.0.3 and ownCloud 10.0.10/Contacts 1.5.5 both handle the custom field Todestag in this way:
PRODID:-//Sabre//Sabre VObject 4.1.6//EN
Android 8.0 contact sync’ed using the ContactSync app shows the custom field Todestag correctly:
Thunderbird 60.5.0/Cardbook add-on 33.9 shows the custom field label Todestag correctly:
IOS 12.1 is also able to handle custom labels although the vcf syntax differs a bit:
PRODID:-//Apple Inc.//iPhone OS 12.1//EN
FN: Dean Death
What about leaving quering of the DNS to the OS, but doing the validation of the answers in the application?
When openssh is linked with libldns is does DNSSEC validatation (evaluation of the SSHPF records). So has openDmarc to be linked with libunbound to do DNSSEC validation.
If an application wants nowadays to do DANE/DNSSEC is has to use the right libraries.