I took a few levels in Hexblade Warlock and when I go to select spells, specifically level 0 cantrips, Hex is listed. However, when looking at the rules, Hex should be a level 1 spell. I see nothing on my characters features that would allow Hex as a cantrip, but I definitely have it listed as an option under Add Spells. Here is a screenshot:

You see how it says “Version” there:

That’s a dead giveaway that it’s homebrew. On of the other players in that campaign must have created it.