You can't "hold down" shift when writing CONSTANT_NAME if you're doing any kind of touch typing. You constantly have to switch between pressing left shift or right shift, so basically you're writing 13 key-combinations. With caps lock, you only need to use a key combination once (for _), instead of 13 times without.

Any kind of touch typing allows ring finger to substitute for pinky when the latter is shift-locked.

Sure, but it's definitely harder than using CapsLock.

Any kind of touch typing? No, I don't agree with that. I can type 140+ wpm and don't look at the keyboard, and I can hold down shift while typing CONSTANT_NAME. Pinky on shift, remaining 9 fingers type CONSTANT_NAME. The pinky would not be involved with this particular constant even if it were free. The kind of touch typing you learned in school has a problem with this, maybe.

A is normally typed using the left pinky, so holding left shift will force you to use the left ring finger for it.

To be fair, on some keyboards, including the one I use, it's easy to reach both the right and left shift keys, so it's easy to alternate shift keys as needed to avoid using other fingers or moving your hand from the home row.

I guess it's important to note that I would use the right shift for this. I have the ability to choose on-the-fly rather than be forced into one or the other by a typing method that I learned in school. I could still do it with the left shift but it's not the one I would choose for this.

It's probably fairly obvious what I think about the taught-at-school touch typing method. It's a decent middle-of-the-road option that's intended to be easy to teach and to work for most people in most situations; that's all.