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

But maybe most players would find it more convenient to use some less obvious keys like F10, F11, etc.
In general, are one-way transfer buttons useful, or should hotkeys be made only for exchanging units?

Since most people are used to F10 and F11, it's probably better to keep them.
I don't use the one-way transfer buttons; as Lider1 said, they often leave a scout stuck in the transfer chain on a slow unit.
Of course, you could add arrow keys for one-way transfers for those who want them, but I don't think many people would use them.

By the way, another question: why does Alt select all units, but Ctrl only splits them one by one? Make Ctrl also split all units at once; otherwise, it gets tedious to click to split units one by one... for battles. And Ctrl+Shift, for example (like in TE), could split them one by one.

But what if you don't need to fill all empty stacks with single units? Often, a large number of single units just gets in the way and slows down the battle due to the large number of necessary movements and clicks on "wait" and "defend." If you only add an extra button that immediately fills all stacks with single units, then it would be appropriate.