It appears we got confirmation of a sequel to Horizon Zero Dawn this week from one of the most unlikely sources, but one worth trusting. During Star Wars: Celebration, actress Janina Gavankar (who provides the voice of Tatai in the game) revealed that there is indeed a sequel and that her comments suggest she's already seent he script and possibly voiced it already. Below is a quote from a Reddit post which had a clip of her comments, but the clip has sadly been removed by Twitch.
She was talking to a fan at Star Wars celebration cosplaying as her Battlefront character Iden Versio, to where the cosplayer mentioned that she got her into playing Horizon Zero Dawn. She then replies "It's incredible, wait 'til you see the sequel. You're gonna die – I know some secrets! – you're gonna die."
We're guessing if she's already at the point of seeing a script to possibly voicing it, that means production on a Horizon Zero Dawn sequel has to be somewhere nearing completion. But with Sony no longer a part of E3 this year, the chances of us seeing a trailer for us are slim to none until they decide to hold their own event. And based on production and what Sony has planned moving forward, it's pretty safe to assume the next game will be for whatever the PS5 turns out to be. But we're guessing we won't know much more until later this year.