Yes, kidney capacity is routinely tested in 60+ old people (at least in my country) as well in pets (my two 12 yo cats are tested every year).
I'm wondering why anyone would disagree with you...
I understand that kidney donation risks are not well-understood, but ignorance permeates virtually every topic here.
Off the top of my head, I'd say that salmonella from lettuce or eating out would be one of the greatest risks to a kidney donor, though for women pregnancy might be high-risk with one kidney. An MRI might also be a concern, because of the risk of allergies to dyes affecting kidneys and the liver.
(The reason I'm shadowbanned is because dang is also ignorant about what science is. My posts are nearly 100% factual, but that doesn't fit the Marxist narrative here.)
Essentially you're halving your kidney capacity, which is not noticeable in your 20s, but will creep up on you later as you age.
It also means that any toxin or infection that attacks the remaining kidney will be an emergency. Women are especially prone to UTI that cause kidney infections.
(My father was missing part of one kidney, and it was on his mind throughout his life whenever he became ill.)
So make sure you really like the person you're donating to, since there are consequences to yourself.
Also, I hate to mention this, but the CCP has halal kidneys for sale, with a replacement guarantee even. I think you know where they're harvested from.