I am literally only talking about 5e D&D games. I am avoiding talking about other games precisely to avoid the “apples and oranges” dismissal.
When playing 5e and only 5e, I can tell when I have more agency in one game than in another and why.
If that’s not essential to this whole discussion, then I don’t know what is.
But isn't agency something we must care about in the extremes? Like when we feel deprived of it, or when we feel empowered? At least for me, it's not a concept I consider much otherwise - I don't sit around grading how much agency I had playing a game.
And again, even if you do know whether one game has more agency than another, another player might feel differently about those two. Not necessarily to the point of reversal, but nevertheless different.
This thread started out with a question about a GM/player conflict related to a player complaining about lack of agency. To me, that's the interesting point about this discussion. Not defining agency, how different systems related to agency or the effect of various techniques. But how to handle a player who feels deprived of agency (assuming the complaint is genuine and not the expression of a different kind of conflict). RPGs are social games. We need the group to have a certain degree of harmony and consensus.
Agency can also relate to different things than just how the GM runs the game. Some players don't feel agency unless they are able to influence other players - some even need to lead. I've had players who needed intra-party conflict to be happy. That's an interesting challenge if there isn't consensus about that kind of playstyle. Others lose agency if the group becomes too big.
So even with the same system, the same prepared material and the same GM - different players can end up experiencing agency differently.
Luckily, my experience is that lack of agency isn't really a common problem. And when I've seen it, it's been related to "domineering" players or GMs more than anything. Such people can be amazing with right crowd (either who enjoy the show or have the personality to compete with such a person), but can ruin the game for other personality types. I've also seen some complaints about lack of agency when things became too easy - "It's all meaningless, we can do whatever we want - nothing matters" or the reverse when it things were too difficult - "When we do well, we get punished with harder challenges - this isn't fun". But I suspect those feelings wouldn't care if they were caused by preplanned or ad hoc GM'ing.