Any moves like U-Turn, Flip turn, Shed tail, or Chilly Reception that switch the user out of battle softlock the game and lock you into the "Choose Next Pokemon to switch in" screen along with the eject button item and emergency exit ability. Can be replicated by using a Pokemon with any of the aforementioned abilities, items or moves and triggering them during a Max Raid Battle.

I believe these moves will also softlock normal battles when you have no other pokemon that can fight.

Using U-turn in a trainer battle with no other pokemon will show "Choose a new pokemon to send out!" with no option, locking the battle