So when did they actually stop manastone usage in the planes?
 

Sounds like an important question.

you could use them in classic PoF

confirmed

( one reason I remember is bc it was the most likely place I could always have regen up due to plentiful druids around and thusly as a wizard use my Manastone more than normal )

I remember clearly using them also in PoH. in classic also.

When plane of sky comes out is when manastone stops working in the planes

Manastone in the planes nerf was end of January 2000.