User blog comment:GoldenPlayz14/Why I beleive that the elemental/upgraded versions of weapons should be there own wonder weapon/@comment-27887623-20181116014409/@comment-1905698-20181116020329

It was discovered toward the end of BO2's support cycle upon which they stop updating the game except for maybe some critical security fixes. In the BO3 remaster, according to this video, the elemental part has been removed; if it was intentional, surely they would have it be fully functional?

Not to mention devs tend to leave bugs in the game if the community likes to toy with them, unless they're harmful to the game. The "elemental knife" is little more than a bug of merging two melee attacks.