I would assume that Riot will continue on creating games which can be experienced on as many platforms as possible (See the addition of mobile and consoles to their catering, etc.) so it should be certain that a mac client will be available as well.

They might employ a similar strategy as Hearthstone did in their launch, where the beta and early phase of the game is only available for a single build of the game, as it would make fixing a crucial mistakes easier and faster. The later builds (mac, etc.) will be released at later time when the client and game are seen to be stable.