I think I know already, but I'd like to get other opinions.

My opponent attacks with a Shiba Yojimbo and any Shugenja. I flip my favorite, Endless Plains ! In a rush of exuberance, I pop it. Opponent chooses to sack the Shugenja, which is promptly cancelled by the Yojimbo. I stare blankly and cry, bewildered, thinking "No, no that cant,... that can't be right!"

Is this legit? Can the Yojimbo do that?

Yep. The Yojimbo doesn't care who chose the target as long as its targeting a Shugenja when the ability tries to initiate its effects.

Never mind. Nothing to see here.

The opponent cannot do that. He has to choose a target that actually has a potential to change the board state. Because the yojimbo is in play and blocks any effect on the shugenja, there is no potential for change in board state by selecting the shugenja; therefore, he must select the yojimbo. It is the same thing as For Shame when steward of law is in the conflict, opponent must bow a character because they cannot dishonor due to steward of law.

48 minutes ago, HidaYama said:

The opponent cannot do that. He has to choose a target that actually has a potential to change the board state. Because the yojimbo is in play and blocks any effect on the shugenja, there is no potential for change in board state by selecting the shugenja; therefore, he must select the yojimbo. It is the same thing as For Shame when steward of law is in the conflict, opponent must bow a character because they cannot dishonor due to steward of law.

I don't think that's correct. The Yojimbo isn't forced to do his thing. So at the time of choosing, the Shugenja is a valid target. It's not until the effects kick in that the Yojimbo's reaction may or may not (and probably will) initiate.

1 hour ago, HidaYama said:

The opponent cannot do that. He has to choose a target that actually has a potential to change the board state. Because the yojimbo is in play and blocks any effect on the shugenja, there is no potential for change in board state by selecting the shugenja; therefore, he must select the yojimbo. It is the same thing as For Shame when steward of law is in the conflict, opponent must bow a character because they cannot dishonor due to steward of law.

The potential of an ability's effect to change the board state is evaluated without taking other abilities into account, especially not cancels. If you had to take cancels into account, Kisada would prevent the opponent from triggering any action abilities during conflicts. It is not at all the same as Steward of Law and For Shame.

2 hours ago, HidaYama said:

The opponent cannot do that. He has to choose a target that actually has a potential to change the board state. Because the yojimbo is in play and blocks any effect on the shugenja, there is no potential for change in board state by selecting the shugenja; therefore, he must select the yojimbo. It is the same thing as For Shame when steward of law is in the conflict, opponent must bow a character because they cannot dishonor due to steward of law.

Consider Hida Kisada. Were your interpretation true you would be unable to play any actions while Kisada were active (as he would just cancel it).

4 hours ago, HidaYama said:

The opponent cannot do that. He has to choose a target that actually has a potential to change the board state. Because the yojimbo is in play and blocks any effect on the shugenja, there is no potential for change in board state by selecting the shugenja; therefore, he must select the yojimbo. It is the same thing as For Shame when steward of law is in the conflict, opponent must bow a character because they cannot dishonor due to steward of law.

This is not correct. The shugenja can be freely targeted which the Yojimbo can then cancel with her reaction.

Isn't using the Yojimbo's ability also technically a change in board state?

Good question. In a situation in which a character has activated a Grasp of Earth, if their opponent tries to activate their Shinjo Outrider to move into the conflict, obviously they can't move in, but the board state now knows that the Outrider can't use the ability again. Is that considered a change in board state, and does the game have a distinction between board state and game state?

1 hour ago, nungunz said:

This is not correct. The shugenja can be freely targeted which the Yojimbo can then cancel with her reaction.

*Interrupt

18 minutes ago, Mirith said:

Isn't using the Yojimbo's ability also technically a change in board state?

Yojimbo's cancel is considered a change in game state for the Yojimbo card itself, but not for another card. A card must be able to have the potential to change the game state on it's own, and you don't take into consideration later card interactions.

Quote

An ability cannot initiate (and therefore its costs cannot be
paid) if its effect on its own does not have the potential to
change the game state.

A triggered ability can only be initiated if its effect has
the potential to change the game state on its own. This
potential is assessed without taking into account the
consequences of the cost payment or any other ability
interactions.

3 minutes ago, AradonTemplar said:

Good question. In a situation in which a character has activated a Grasp of Earth, if their opponent tries to activate their Shinjo Outrider to move into the conflict, obviously they can't move in, but the board state now knows that the Outrider can't use the ability again. Is that considered a change in board state, and does the game have a distinction between board state and game state?

Board state isn't a thing, just game state.

2 hours ago, AradonTemplar said:

Good question. In a situation in which a character has activated a Grasp of Earth, if their opponent tries to activate their Shinjo Outrider to move into the conflict, obviously they can't move in, but the board state now knows that the Outrider can't use the ability again. Is that considered a change in board state, and does the game have a distinction between board state and game state?

its stated that if a character cannot complete a move then its an illegal action and cannot be done in the first place. So outrider wouldnt be able to even active her action

Move Some abilities allow players to move cards or tokens. ◊ When an entity moves, it cannot move to its same (current) placement. If there is no valid destination for a move, the move attempt cannot be made.

11 hours ago, AradonTemplar said:

Good question. In a situation in which a character has activated a Grasp of Earth, if their opponent tries to activate their Shinjo Outrider to move into the conflict, obviously they can't move in, but the board state now knows that the Outrider can't use the ability again. Is that considered a change in board state, and does the game have a distinction between board state and game state?

The fact that an ability has been used is not a change caused by the ability's effect , so it cannot be taken into account in the legality check, just like changes caused by paying an ability's cost.