This is something I've been thinking about a lot in relation to my own games. The question is simple, and right up there in the topic. The question comes down to how much control do you give to your players? Does it change with the different kinds of games you may run? Obviously the ideal is to give almost total agency to your players, or is it? Today I want to talk about that and the kind of games it shows up in.
The Plot Game
The plot game is one where the GM/Players are trying to tell a specific story. Maybe you are trying for an epic fantasy story, maybe you are trying to tell the story of a particular war, or maybe you are just trying to tell the story of one person's quest for the ultimate bagel. It doesn't really matter because what does matter is the fact that the game has a planned end point in it. That end point, and the fact that the game is meant to be working towards it, means that there are going to be times where agency needs to be curtailed. Generally this will give the game - in terms of player choice - that the game has a diamond pattern. At the beginning choices are locked down a bit more to get things moving in the right direction, but as the players move forward they get to make more and more choices. Eventually though, at the point where the most choice can be made, the players set on a path and from there get less and less ability to deviate until they reach the end of the game.
Now, technically, this game still has a lot of agency. There should be an agreement between players and GM beforehand to run this kind of game. The players are choosing how they get to that end, and they do get to make the core decisions. However, some decisions just can't be made without breaking the game. Or, well, they can but it is going to have consequences. Like "the urukai march unchecked into helmsdeep" consequences. That is a choice that can be made though from a player's perspective it basically looks like a "this way, or death" which can be a problem if that isn't what people agreed to.
The campaign type game - what RPGs made their name with really - is different. In this kind of game almost everything can be down to player choice. The point of the game is to adventure, relax, and explore the world. As such, in this kind of game, the players can - and should - be the ones making all the key choices. This is basically the best kind of game to do the "training wheels are off" sort of things and just see what happens. This doesn't mean that there shouldn't be consequences for those choices. A choice isn't a choice without consequences, but the scope of those choices should stay wide open. There's no end point being worked towards with this, so there is no reason for those choices to be limited as the game goes on.
The hybrid is a strange kind of game, but when it comes to agency it isn't much different than you'd expect. If the plot game is a diamond, and if you picture a campaign as a square, then the hybrid is a pentagon with a more square base. The only question here is if you do the aimed start, or if you start full campaign and then the players - or you - turn it into something with a dedicated ending to the game. If you are transitioning into a plot you want to warn your players that their breadth of choice is going away. Why? Well, if your players are used to just being able to do whatever they want, and suddenly there are hard in world events that restrict those choices, it can feel like you're being railroaded into something. That isn't a good feeling, and can really ruin the momentum a game has had as suddenly a player isn't as willing to play along. Alternatively, it can be hard to bring that plot to bear if the players keep running away from it because they're used to just being able to do what they want.
On the other hand, the amount of choice that suddenly becomes available - even if eased into it - when you start with a more plot-based beginning can also curtail the feeling of freedom a campaign is supposed to have. The players will figure this out eventually, but you also want to make sure this is the kind of game the players want. Afterall, it can be unfun to make a character for a big story, only to find yourself in a more campaign experience.
Finally, there is the last kind of hybrid which is very popular for console and computer RPGs. This kind of RPG starts with a plot beginning, ends with a plot ending, but int he middle there is a large portion of time where the game works like a campaign. In a way, the story of the 47 Ronin is like this. The storied beginning with the 47 serving their lord, and then the lord's death is the beginning. The years where they wander and prepare would be a more campaign type setting. The "game" then transitions back to plot when the Ronin decide to avenge their lord and stage the assault, ending with everyone committing seppuku after vengeance is had. I mention this as a possible way of doing hybrid, but depending on the length of the 'campaign' portion of the game, it could just be a plot type game with an extended middle.
So How Much?
So how much agency should you give? Honestly, it varies. It will vary by group, and it will vary by game. The key thing to find is just how much agency can you give to your players without compromising the core feel of the type of game that you all agreed to? The big point there is the type of game that you all agreed to. Being clear on those expectations will make for a better and smoother flowing game. That said, finding the right amount of agency can be hard. I generally feel you should err on giving too much and then reign it in as you feel things slipping away. Just, and as always, keep in communication with your players about it. It can really help a lot.