Is it common for a player not to know they want to take an enemy alive before combat? I usually just tell my players that if they don't want to kill am enemy they need to declare it before hand. Granted I haven't had it come up a whole lot but that's just how I deal with it and thought that was universal.
It is less that they don't know (though maybe they don't at the start of combat and figure it out part way through) and more how the game used to play versus how it plays now.
It used to be that non-lethal attacks were declared round by round, because they were a special sort of attack. This meant that if a player forgot to declare their attack non-lethal, then the DM could declare it was a lethal attack. And, actually, depending on the DM, this may happen anyways. I was playing a game and the player declared a non-lethal attack to take down a commoner... but they crit. And the DM ruled that since they crit, they amputated both of the commoner's legs and they were bleeding out. Because the damage was too high.
Now, the game rules could have allowed you to, at the start of combat, declare certain enemies were to be taken alive, but this causes some issues. It is possible the enemy you picked to take for questioning runs away, and now you don't have the option to take the remaining people alive, because you didn't declare it. Or you declared it and now you don't have the option to kill the enemy if something is revealed mid-fight that changes your mind. There are just a lot of potential edge cases.
So, instead, the game has set-up very permissive state. When you finish attacking and dealing damage, you can make the decision. This means you can react to a changing situation with ease, and there is much less chance of "well you didn't
say you were attacking non-lethally, so they are dead" which became so common.
And if the only cost of that flexibility and ease of the players being given the option to spare the lives of their foes is that the description of an attack might be retconned before the end of the player's turn? I'm fine with that. I'm more than fine with that. Because the benefits outweigh the negatives by orders of magnitude. After all, an attacks description can change mid-attack (divine smite) so it isn't like there is no precedent.
