Just noticed that there is a similar question on the MTS community forum posted yesterday evening. That person too is going to discontinue the long distance add on due to lack of use.
Usually price changes are put in the news section of the MTS bill and I went back through a few months of them and didn't see anything about this change.
As its an optional service maybe MTS doesn't have to say anything in advance. It will be interesting to see what the MTS official answer is.