are there any plans to add no items subcategories for the categories where that's possible?

the mode doesn't exist in game

Yeah, Mario Kart World's VS Race only has options for Normal and Frantic items. All Time Trials could be a category extension (with another category for All Time Trials NITA).

There is a weird workaround to play VS Race with no items though. 30 Tracks (1P2C NITA) would be 1 Player 2 Controller with COMs off, so you could race with one controller while using 0 items. Quitting once 1 player finishes the race is allowed, so while you would have to quit out after every race, you can play VS Race with no items this way.