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:
BEGIN:VCARD VERSION:3.0 PRODID:-//Sabre//Sabre VObject 4.1.6//EN UID:2ba8a291-5cc9-4865-a9c4-4d053a0ab080 FN:Dean Death BDAY:19500101T000015 DEATHDATE:20181231T000000 END:VCARDAndroid 8.0 contact sync’ed using the ContactSync app shows the custom field Todestag correctly:
20190203_android_8.0_deathdate_display.jpg
Thunderbird 60.5.0/Cardbook add-on 33.9 shows the custom field label Todestag correctly:
20190203_thunderbird_60.5.0_cardbook_33.9_deathday.png
IOS 12.1 is also able to handle custom labels although the vcf syntax differs a bit:
BEGIN:VCARD VERSION:3.0 PRODID:-//Apple Inc.//iPhone OS 12.1//EN N:Dean Death;;;; FN: Dean Death BDAY:1980-01-01 item1.X-ABDATE;type=pref:2000-04-30 item1.X-ABLabel:Todestag END:VCARD