It's been on for a long time, then. I remember even before the lockdown, I would wonder why Little Christine had algebraic equations to figure out her age, when her birth date was right there for anyone to figure out her age with simple subtraction.
And if the field is something that can be accidentally toggled on, then that's even more reason for people to be concerned about providing their specific date of birth. A lot of us leave a lot of bread crumbs as to who we could be and generally where in the world we live based on our posts over time. Paradoxically, for the vast majority of ABDLs, privacy and discretion is very important to them. Those that want to identify themselves in public that they are ABDLs can do that right now. But for me, I prefer to keep my own counsel. And one thing I don't want is for everyone coming to a diaper fetish website to have the ability to know my actual date of birth.
Looking at my profile, other items that I wouldn't want a lurker to know include my email address, and the last 3 months of where I've logged in from. And I certainly don't want that information available to be obtained by a hacker, either. Nor do I want a forum software update to unknowingly make these fields viewable by the public for months at a time.
Taking a look at the Wayback Machine, I can see that the date of birth was publicly viewable included at least as far back as 2015.
Perhaps you can provide specific reasons as to why you need to know a user's date of birth and to keep it in your records. Because as far as I can tell, the main thing you need to know is if someone is over 18 years of age. Once that is established, there's no need to keep the actual date of birth; an estimation should be fine. I don't list my DOB in Facebook, Reddit doesn't need to know, and I don't bother with Adisc.