Where is it acceptable to mention an abnormally high stipend?
Salary negotiations are about supply and demand.  If your stipend has come to an end, it is no longer relevant to that supply and demand.  If your stipend is ongoing, you can say you will only accept a new position if it pays more than the stipend.
Your past stipend pay is not very convincing evidence of your merit.
Your past stipend pay is not relevant to the selection of PhD students.  Discussing money may give the impression you would be more suited for an industrial position.