Up until I was about 19 or 20 I could sing two different notes at once. I didn't know it was a thing most people couldn't do, I didn't know about Inuit throat singing/overtone singing, it was just something I'd do to amuse myself. I'd always been a weird kid and it just seemed like a par for the course ability.
At some point I stopped doing it - I think around the time I moved to the city. Living in a rural area always ensured hours of solitary driving time that I could use to loudly practice.
I can't do it anymore at all, and every so often an article will pop up about it and I'll kick myself a bit for losing it. Whatever was going on with my vocal chords then doesn't exist anymore.
Also one time I looked in the mirror and realized a big mole I'd always had on my forehead was gone. I was actually drunk when I made the discovery and excitedly kept trying to tell my friends about this mole I used to have. It probably goes without saying that they didn't give a shit.
Moles aside, what powers have you lost over time?