(read the tittle) that way you can sneak and use the shield without trigger the katana special attack.

Yes this would be very nice same for the rapier imo. Or possibly be able to bind the special to another key would be my preferred way?

i think both solutions are good, they should implement both, maybe they should add my solution first and then make your solution an configurable option, something like "special_attacks_require_a_keybind_to_excecute:true". I prefer this because sometimes when you have a big modpack installed you ran out of keybinds, that's why i prefer that the especial attacks gets blocked when sneaking, that way you don't waste keybinds, but i know that some people would prefer a keybind to trigger the especial attack, that's why it's better to implement both solutions.

Actually, I thought not being able too use offhand was katana's downside, to make up for it's crazy high damage

Pretty sure katana damage got nerfed for next update.