In Winter Soldier we see him going toe to toe with multiple Avenger-level people including Black Widow, Falcon, and Captain America himself. Obviously we can attribute this to his training and status still being within HYDRA. In Civil War, the 1st half shows he still has incredible combat prowess and superhuman speed -- in the apartment scene where they face off against German counterterrorism units he is shown to be crafty and able to take on multiple combatants, and this is when he is no longer being controlled by HYDRA. But in the 2nd half of Civil War, he suddenly appears weak -- he gets somehow overpowered by Spiderman, takes ages to run to the hangar with Captain America (did the movie forget how fast they were moving through the highway during the previously mentioned scene?). In the final fight with Tony he's had to almost rely on Cap for all the fights and appears extremely weak. And in almost every subsequent movie including Infinity War and Endgame, he's been reduced to using a gun and just being really toned down, or "nerfed".
In the most recent TV series, Falcon and the Winter Soldier, he appears even weaker, taking on sub-combat roles and never taking charge once or showing that same battle prowess he had in Winter Soldier and the start of Civil War. In fact, the only time it ever seems like he regains those fighting skills is when he's pretending to be the Winter Soldier for Zemo and Falcon to maintain appearances.
Is there a canonical reason as to why Bucky got "nerfed" throughout the MCU? I've been a huge fan of Bucky's character throughout the MCU so it's been incredibly noticeable how much weaker he's gotten in each subsequent film from Winter Soldier.