They quit using aluminum wiring back in the 70's. Not that I would use it but are the smaller sizes still avialable? Is it legal to use? How come it is still used for ranges, panel feeders, etc?

Last I talked to a manufacturer rep; small gauge AL has not been manufactured for many years.
You can get SER and other cables in # 8 and larger
THHN comes in #6 and larger

As to why, I have to assume it's cost effective. We don't see it here except for feeders in residential. Sizes #1 and larger.
Seems to be out of vogue in commercial at the moment.