So, I think you're bang on here. The exports reveal that the info is included like so:

item1.X-ABDATE:20160302
item1.X-ABLabel:Death

My cursory searching tells me that this isn't outside of the CardDAV/vCard spec per se, but that the X- signifies "extension", aka basically, info that is being included but may only be read by apps that specifically know to look for it.

Now I need to decide if I'd rather have them show up as death anniversaries on my android calendar but not show up at all on my desktop clients, or redo those dates to be maybe a note or something though then they won't sync to a calendar, or remove it entirely from the contact and instead just have it as an event on my calendar that repeats. Hmm...

Thanks for the idea though, it got me to look at the file itself which was very helpful!

Interesting, I hadn't even considered that the apps could have been doing something other than CardDAV. I'll have to look into this angle more. Damn shame if true.