I think if you are planning to cook for someone, or take them out for a meal, as a host it's an extremely good idea to first check for dietary needs. As a guest, if I was invited somewhere, I would provide the information off my own bat if I was not asked.
The result if you don't could be a ruined evening, and if allergies are involved an unpleasant or life-threatening one.
But I would say that for some vegans you can't just say it stops when you aren't going to be eating. It depends whether the person is a "whole life" vegan or a dietary vegan. But I cannot fathom why it's relevant for us as viewers to know someone is vegan.
It might be relevant for it to be brought up with an employer if there would be moral or ethical reasons why something could be problematic for you to do if asked by your employer. That could be presenting hours with non-vegan friendly brands, or sampling non-vegan foods, or wearing non-vegan clothes or shoes...
I've noticed that Charlie - who doesn't drink - won't eat or drink anything made with alcohol, even if it's no longer got any alcohol content. I think Jill Franks is the same. Both do actually state that they don't drink as a prelude to declining to sample. I think that's sensible in the context of presenting a food and drink show - they are making sure that there is no implication that the proffered food/drink item is horrible.
But you don't need to announce these things every time you draw breath... unless you want to be accurately labelled an utter bore on the topic.
Just my point of view.