I just thought I should come back and update this--Joanna wrote and said that she had gone to the Coke site, and Diet Coke with Lemon was all over the place, so it didn't seem likely it had been discontinued, and then I realized that the last time I bought it, about a week ago, it came in those weird new long boxes--"fridge" boxes or something--so it also didn't seem likely that they would be bothering to repackage it if it was discontinued, so it seems that the rumor was false.

It may be a regional thing.