And Magneto never says his name among the mutants Trask killed in Days of Future Past. "Angel. Azazel. Emma. Banshee. We were supposed to protect them!" If Magneto is willing to honor his enemy Banshee this way, why not Riptide? It's pretty clear Trask killed him too yet apparently you didn't consider him worth mentioning.
Riptide and Azazel are my favorite villains in X-Men and while it would have been cool to see them again atleast they gave them the dignity of a definite final fate rather than disappearing for no reason.