HoMM3 HD is a mod (add-on) for the game Heroes of Might and Magic 3.

Let me try comparing the quick cast bug with others using specific examples...
But we definitely need to address the "smart" quick cast, and not just mention it, but convince the player that the old quick cast, which they're used to, is a bug, and the new one is the correct version. If I forget to mention it and they lose in a match, it will be my fault. If I mention it but can't convince them, we'll play without the new quick cast and without some other useful features.

If the other player is using a mod, there's no need to explain anything. The player is using a mod, which means they're ready to use all its features. At the beginning of the game, you'll have a list of agreements. (I anticipate that this list will grow over time. But I think there will be an option to group options in the future. This is one of the concepts I was talking about as regular, i.e., use whatever you want, but agree on it. In the end, the log of starting options should be identical, and it creates the same game conditions so that no one has questions later.
And if you're playing Heroes "without bugs," and you want the correct version, then there's even less need to explain anything. This is a different concept, truegame, for playing Heroes without all the bugs. It's clear that bugs will be fixed over time. And eventually, the player will come to that source, I hope.
And in a game between a mod user and a SoD (Save and Load) user, the ability to undo a quick cast will not be available to you. This is the third concept - controversial, difference version. The most debatable. Although, if you think about it... People play in different competitive environments. One player is more equipped. Whether to allow or prohibit the use of certain mod features is not so straightforward. There's 2.98 with the display of the number of mana points, and there's RC5 with the undo quick cast, i.e., no one can prevent playing with these features. But if you look ahead, it won't really affect anything. If the SoD user doesn't switch to the mod, then, in theory, they don't intend to compete with the latest version. And there's no need to worry about unfair treatment towards them. Just with the further addition of features to the mod, the mod needs to detect that the opponent is using SoD and set the corresponding settings.

And what is all this for? To fix a bug that is quite rare in the real game, has little impact on the game, and is familiar to everyone, and many don't even consider it a bug.

It's not quite clear what the conclusion is, but I see some contradiction. If the bug is quite rare and has little impact on the game, then there's no harm in fixing it; no one will notice, and no one will be disadvantaged. ;) And why is there so much fuss about quick cast? And to be more precise, about the correct quick cast. But otherwise... there will be one less bug. We're striving for a game without bugs, or should we also use bugs because we like being deceived?