Why would you need to learn birthdays (except maybe closest family)? It's maybe the canonical thing to put into a calendar.
"Because I want to" is obviously fine, but it still reminds me of the infamous "capitals of the world" flashcards that spaced repetition practitioners warn against.
It wasn't until I discovered Anki a few years ago that I finally was able to remember my parents' and sisters' birthdays (despite calendar reminders every year).
Why remember something that can be looked up? Latency; it's super useful to have things appear in your brain so you can immediately do something useful with them instead of slowing down and looking something up and potentially losing context.
So, Anki did help me learn a few of them, while a few would just never stick for some reason. Amusingly, the few birthdays I did learn didn't really help me because even though I knew the birthdays, they wouldn't come to mind until days or weeks after the birthday passed!
So what I'd actually need is "what are all the birthdays that occur this month", and then have a ritual habit installed that at the first day of the month I would spend a few minutes pondering this list in my mind.
(Again, the birthday wall calendar would probably work better here...)
For me specifically the motivation was that my mother, who is naturally very good with dates and birthdays, would keep telling me "It's your {uncle, grandma, cousin}'s birthday, don't forget!" and I would think to myself, "what kind of bad person am I, that I don't even know my own family's birthdays?"
Similarly, I struggle with understanding/remembering the structure of my family tree, at least for the family members that I never met (I am often told stories about ancestors, and am shamed for my inability/unwillingness to naturally and automatically memorize them). So I broke the family tree up into individual relations and made Anki cards of them.
That worked a little better than the birthdays, but still a struggle, because it is "meaningless information" as far as my own brain is concerned. I might consciously think it's important to have knowledge of your ancestry, but my neurology apparently disagrees... Alas!
Don't bother. Do you want your distant relatives to toil away trying to remember where you fit in the family tree? Life is too short for that. Forgive yourself and move on.
I'm bad at it too and my wife is not, so she can figure out my own family tree better than I can; it makes for good party banter.
Why are spaced repetition "practitioners" warning against that?
I have a "Roman Emperors" Anki deck. It serves absolutely no purpose except making me feel good about it, but it works well (except maybe for the final few emperors, those tend to be leeches).
If you're truly interested in Roman Emperors, that's fine, of course. The warning is more about "I just started spaced repetition, I need cards quick, so let's do capitals, US states, and other random trivia".
I admit birthdays of acquaintances don't quite fit under "random trivia", but I'll still maintain that learning random birthdays seems useless, when the calendar works so well.
"Because I want to" is obviously fine, but it still reminds me of the infamous "capitals of the world" flashcards that spaced repetition practitioners warn against.