Not a cryptographic expert by any means but maybe something like this would work. This’d be implemented in common places people shop: supermarkets for instance. You’d go up to customer service and show your ID for visual confirmation only; no records can be created. In return the service rep would give you a list of randomised GUIDs against which the only permissible record can be “has been taken”. Each time you need to prove your age you’d feed in one of those GUIDs.
Honestly I don’t care what you want to eat. It’s your body, you can shove whatever you like into it.
Where vegans become a problem is where they’re being evangelical about their beliefs and trying to force their audience to feel the same shock and horror as they feel when contemplating the meat industry. If all I hear from you is restricted to when I offer you food and ask if you have any dietary requirements, and is of the form “I’m vegan”, that’s absolutely fine. If we’re friends I’ll adjust the menu for you, although you might have to accept it’s only your plate that gets veganified.
You going “eww” and talking about “rotting corpses” or whatever is where it becomes a problem. If I’ve asked, obviously I’ve brought your response upon myself but you should still tone it down for the non-vegans. “I’ve looked into the meat industry and I didn’t like what I saw” would be a good first response; make sure not to release any gory details unless people are really pressing you for that level of detail.
That said, none of this is based on actual experience of offensively evangelical vegans. I’ve heard they exist but haven’t met one yet. I’ve known some people for quite a while before finding out they’re vegan, veggie or whatever.