The assist shows that you are helping your team to get picks or in the overall match, which adds rating points after match ends.
Till now the assists showing ui qas just a small character shown in the kill feed, but with the new update and new act the assist ui adds a little overlay at the center of the game screen. This new assist ui may make sense or help some of the users, but may feel it annoying and distracting, as the ui is just showing at the center and its quiet big to act as a distraction.
Even when you are in a fight or 1v1 with you opponent the ui can appear at the center which can interupt your aim. Due to this distraction, Riot should allow user to turn on or off this assist UI according to their interest.
Turn off Assist UI
Well there is not option to turn off this assist ui appearing at the center. But with the concern of the players, Riot may bring the toggle soon. We will post the update and guide here.