Yes, Combo Assist is permitted in the professional scene, though it's rarely used because at that level, it's limitations hurt more.
In
Killer Instinct, all it does is:
- Removes the need to perform directional inputs for cancelled Special Moves in the Combo System
- Permit the Player to do an Ultra Combo simply by pressing Forward and the three Punch or Kick buttons, depending on the Character
- Allows the Player to perform two specific Enders in the Combo System by holding Forward and pressing Heavy Punch or Kick
- Prevents the player from using their character's Combo Trait unless they hold "Back"
- Makes a player's Combos a bit more predictable since it tends to use a specific set of Special Moves
Overall, the major thing it does is help prevent the Player from dropping their Combo during the Combo Sequence. A professional player rarely drops anyway, and instant access to their Character's Combo Trait is of more benefit to them.