So I gave my definition for agency way up above. That most of the time people can make informed decisions based on what their PCs know. The deeds and actions (or lack therein) has an impact on the fictional world they reside in. That is kind of a minimum for player agency to me. For many published modules, this is likely the only level of agency that players really have, The PCs have to stop Tiamat from rising, it's established at the beginning that if you want to escape Ravenloft eventually you have to take on Strahd.
Next, we can add more agency depending on style of game in that the players decide what goals they are going to pursue in that fictional world. Do they even care about that dragon that might destroy the city? Do they want to join the people fleeing the city and strike out for new territories? Who do they align with, who do they help or ignore? Those are all things that can happen in my games.
Next step? Do you allow the PCs to do literally anything in that world, even if it includes murder, mayhem along with all sorts of evil acts? I draw the line there because I wouldn't enjoy running that game and I know that at least one of my players wouldn't want to participate.
Some people want even more agency than that where the players add significant lore to the world or change the world outside the influence of their characters. This isn't a traditional aspect of D&D for most people, it is standard for games based on Powered by the Apocalypse.
The next logical step to me, which isn't really a game IMHO, would be that the players just do whatever they want with no restrictions whatsoever. It's story time, perhaps a collaborative story, but a story nonetheless.
What level of agency works is up to the group. Right now I'm playing Curse of Strahd. I know that if I want my PC to survive I'm going to be taking on the Dracula wannabe at some point. But we still get to decide where to go and what to pursue, I'm still having fun playing the game.
Oh, and of course I'm sure there a a lot of nuances and variations of what I've outlined.