Valorant assist feature is not new, when you give a opponent is eliminated by your teammates using some of your abilities or you gave that opponent damage of 50 or more that elimination is considered as your assist. Though the kill is not counted on you its for your teammate obviously.
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 the update assists showing UI had 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 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.
So the new update had fixed the assist UI a little bit it now not appearing near thw center but on bottom center which is better than earlier, and does not act as a distraction (not that much).
Turn off Assist UI
Well, there is not an option to turn off this assist ui appearing at the center. But with the concern of the players, Riot may bring the toggle soon. Well so the ui cannot be turned off, but Riot had moved it to a little bit lower which is less distractive.