As far as we know, the quest system in Horizon is unlikely to be in the veins of Witcher/Mass Effect consequence-multi decision style. The reason is because in all of what's been told to us about Horizon, they've never once said that the quest system has deep branching paths and consequences.
What we do know, is that there are major side-quest in each village/town, layered with smaller side-quest.
I don't know about other people, but even though I'm quite excited for Horizon, I'm excited in the veins of what GG has said this game will be. Stuff that they did not hint at all, (like consequence, deep side-quest) are not components I'm expecting in Horizon at all.
And no, just because the quest system has no consequence/decision trees doesn't somehow disqualify it as an RPG, because JRPGs are largely that way with quest.