I don't believe many people would say "in Chess the object is to capture the king, not achieve checkmate." I don't think anyone would say that. But that is what Abbott said about Ultima, as a direct quote.

The extra clarifying statement he added at the end ("not achieve checkmate") specifically denies the possibility that he meant "this is equivalent to checkmate."

If the rules of Chess stated "in Chess the object is to capture the king, not achieve checkmate" then that is what would happen. Players would likely still resign before the game is over, as often happens now, but someone determined to play the game to completion would have to get the enemy king off the board to win. I think the wording that was used makes this very clear.

The examples Abbott laid out show that even when the king is guaranteed to be captured on the next turn, play still continues with both players moving normally. If the "winning" player doesn't perform the capture, then they don't win, and the game would continue until one of the kings was captured.

It seems that Abbot suggests the latter.

Could you elaborate on this? Apologies if I'm overlooking something, as I may have parsed through everything Abbott has ever written at this point, and it's a lot to keep track of.

From what I've seen, in all 4 instances where Abbott published an explanation about the rules of Ultima over the span of 42 years, he has always said you can capture the enemy king directly. In none of them does he suggest you're allowed to undo this outcome if it resulted from an illegal move, nor does he otherwise imply you have the right to redo moves without consequence. Again however, if I've overlooked something please let me know.

The only practical issue is what should happen on an unforced king sacrifice: does the player lose through having made an illegal move, or should he take the move back and play another?

I'm not seeing anything Abbott has said that outlines special considerations for someone ending their turn with their king in an attacked position. If there are no rules addressing it, then presumably it would need to be resolved either by the rules or by the players making something up themselves. If it's determined by the rules, the only rule that seems related at all to this scneario is the first one: you are allowed to capture the enemy king.

Looking at Shogi, the 将棋連合規定 offers a similar resolution: If a player's move leaves their own king in check, and their opponent points it out, the player immediately loses the game. The outcome of this scenario is mechanically equivalent to the opponent noticing the king is being attacked and capturing it.

Then again, on the chessvariants page for Shogi, I don't see any mention of how these types of situations are resolved. Presumably it is left as an exercise for the players to decide how they'd like to resolve a rules breach when it arises. Perhaps that could be assumed here as well.

All I know is that throughout his entire life, Abbott has been very consistent with his choice of wording for what "the object of the game" is in Ultima, and yet in our "object of the game" section we aren't respecting that wording. We're actually saying the exact opposite.

When a game's creator states "the object of the game is to capture the king" multiple times, and eventually elaborates to specifically say "the object of the game is to capture the king, not achieve checkmate," it seems inappropriate to write that the object of the game is to "achieve checkmate."

I'd hope we could just write what the creator said, instead of the exact opposite of what he said. His own words aren't going to misrepresent him.