A choose-your-own-adventure or fighting fantasy gamebook can be
interesting. That doesn't mean that I have any agency, though!
I repeat: if all the players are doing is pressing buttons on the GM's black box, they are not exercising agency.
I don't think they're terribly salient, other than the awareness of a decision which I think is a pretty low threshold to get over.
Player agency consists in the capacity to shape the shared fiction in ways that reflect the players' visions and aspirations for that fiction. Given that the main way players impact the fiction is by declaring actions for their PCs, if the players are to have that capacity then they must be able to establish what is at stake in those action declarations. And as
@chaochou has posted, this also requires stable and transparent rules: otherwise what might seem to be at stake isn't really at stake.
There are a variety of ways to achieve player agency, because there are a variety of ways to enable the players to establish what is at stake, and a variety of ways of resolving action declarations in stable and transparent ways.
But some common approaches to RPGing - "quest givers", consequences established by reference to GM's secret backstory, the resulting blind choices, etc - are obviously at odds with player agency. These are techniques for low-player-agency RPGing, and they generate a different set of demands on the GM.