My touch typist teacher said RIGHT. Never considered the left.
Different AC here. Basic non-ergo Keytronic layout. I use left hand, not right hand, and I was taught touch typing (and can still do 100wpm) by a teacher who taught by the book that says "right-handed."
Even though the "6" is, properly speaking, in the "6/y/h/n" vertical row that "belongs" to the right hand, I just looked closely at my fingers on the actual physical keyboard on which I've typed for 10+ years, and its clones on which I've typed for at least 20+, it's because the "6" is closer to the left index finger than the right index finger. The pad of my hand (not the wrist, about halfway up the pad beneath my pinky finger) rests on the lower edge of my keyboard, and my thumbs rest so comfortably on the spacebar that the spacebar has a little worn spot on it.
Home exercise: Place fingers on home row. Touch right and left index fingers to "T", "Y", and "R". For my fingers and keyboard, "Y" is the most comfortable, almost dead-center. Repeat experiment with "5/6/7". For my fingers/keyboard, I can't reach "5" with right. I can't reach "7" with left, and "6" is reachable with either, but more easily reached with left finger. with left on "T" and right on "y" almost centered beneath "6", left is visually confirmed closer to "6."
(Side note: Both by size of wear spot and by observation while typing this post, I almost exclusively press the space bar with my *right* thumb. Maybe that contributes to using my left idex to hit th 6 key -- my left thumb is basically unused. I just typed this entire sentence with my left thumb crammed under the keyboard and it felt comfortable. Undoable with right thumb in equivalent positon.)
P.S; Our touch-typing teachers taught us the same way, but for me and my keyboard, we cheat on the "6". I've forgotten whether it's supposed to matter which thumb you use on the space bar, although I imagine I could have squeezed out a couple of extra wpm if I'd used both thumbs in high school.
P.P.S.: Fuck the last 5 years of UX "professionals" who think everything has to change every six months for the hell of it, or the last 15 years who think that menu options should change depending on which options the software decides are more frequently used. Neither group knows anything of muscle memory because neither group has been in the industry long enough for it to matter.