How do you know that they shall not be, and why? Even contacts/README.md at 73ef6e258f363a9bf7f44cd0e79bdff0b44450b4 · nextcloud/contacts · GitHub is merely an indication that some item types would be difficult, rather than a ban on all new ones.
I ask because Ability to have custom labels · Issue #27 · nextcloud/contacts · GitHub doesn’t provide this functionality — it’s per-contact, rather than a common preset, which the server understands as not being per-contact.
However, it obviously does not fullfil that purpose. Potentially, it could be close if the user stored pure JSON in their notes, but that’s unreasonable.