I ticked 1994-1998 based on my own confidence about who is Gen Y and who is Gen Z. For what it is worth, I have a copy of a book about Generation Y and have searched both physical books and the internet for information on generations and their birth years. This is just my opinion, but it is a conclusion that I came to after research:
I am 100% sure that those born in 1991 are generation y.
I am 99% sure about 1992. And the only reason I don't say 100% is because of the very happening that you have just mentioned. 1991 is not considered a cusp year but for some reason 1992 has not been likewise gate kept out of the cusp/generation z conversations. If people had left 1992 alone, the 1% of doubt would not exist.
I am 90% sure about 1993. I am confident that they are, but if 1992 is sometimes lumped into cusp conversations, then I am going to be less confident about 1993. 1992 is the last generation y year according to Canada, which would make me hesitate to exclude 1993 borns from a gen y/z cusp discussion.
I am more confident than not that 1994 is generation y but in my understanding, 1994 and onwards must be part of any cusp discussion.
1995 and onwards are generation z according to my research and I see no chance for anyone under the age of 25 to be considered generation y.