I have an old Gary Fisher Kaitai (it was first an mtb, but they used the name for a hybrid after that) I bought at a swap meet a few years ago. I don't ride it much, though, I'm a roadie and rarely make it to the trails.

I tried to take it on some mtb tails in Durango, Colorado when I visited there in 2018. I live very close to sea level, and Durango is at 6,000 ft. My attempted ride did not go well. I was barely a quarter mile up the trail (a trail marked "easy"), bent over the handlebars, wheezing. There's just no oxygen there! I couldn't even pass a guy walking two dogs. I still laugh at the memory.