That story is apocryphal to the best of my knowledge. The biggest factor was that Jennifer Lien was struggling with some severe mental health situation and was struggling through the grind of TV production by self-medicating. Combine that with sagging ratings, the writers' inability to figure out what to do with her character, and the inclusion of Jeri Ryan in season 4 as the new sexy character and her removal from the show was all but inevitable. It breaks my heart because Kes deserved better.
There's some sort of improv corollary to this thesis as well.