While it might be lagging an entire year behind the PC release, the Linux version of Hitman has now finally arrived. Much like the majority of Linux ports these days it has not been made by the developers themselves, but rather by Feral Interactive whom you might know from their work on Total War: Warhammer, Tomb Raider, Deus Ex: Mankind Divided, etc.
As for the Mac version, I'm afraid I couldn't find any concrete information. However, Feral Interactive has previously stated that they are waiting for an all-important Metal update in order to start working on Mac versions for their other games, so chances are the same applies to Hitman as well.
If you already own the Steam version of Hitman you don't need to worry about buying another copy, the Linux version will become automatically available to you. And finally, for those among you that know very little about Hitman, here's the most recent Linux trailer to hopefully clue you in: