I started playing Horizon: Zero Dawn a few weeks ago and have been focusing mainly on sidequests and exploring the world. I advanced the game just enough so I got to Meridian, but as I collected more and more quests from NPCs, I started wondering if any of those tasks (or any quests in the game) were time sensitive or could become unavailable after advancing the main storyline. Are there any time sensitive quests at all in Horizon: Zero Dawn?
There are no time-sensitive quests in the game. There also isn't a real "point of no return" for the side quests, after you finish the game your game is reset to the point before the final mission and you can continue to explore and do sidequests.
There is a point where the game warns you to finish any unfinished tasks, but that only means that you are about to do a pretty long quest that you can't leave until you finished it.
There is an order to the three main quest series, if you do them out of order at some point one quest line will pause until you have done some of the other main quest lines.
There is no time gating to quests, but depending on where you are in the main quest characters may talk to you differently when giving quests. If you're later in the main storyline, side quest givers may talk about how honored they are to meet you or wonder why you aren't off doing something more important.
Similarly, when it comes time for the final battle, who shows up and what they say to you depends on which side quests you've completed and in some cases which conversation options you've chosen. Even characters from the Frozen Wilds DLC can show up.