In Avengers: Infinity War, why didn’t Thanos keep his armor on or why didn’t he put it back on? He wasn’t finished retrieving the Infinity Stones so he knew there was still work ahead of him. And keeping it on would be more inline with his character in the comic books.
Let's go with official words of Joe Russo himself:
Speaking to IGN, Infinity War's co-director, Joe Russo, explained why Thanos is dressed down for the battle ahead. "He has a more casual approach in this film and it's very philosophical for him. It's part of his character definition. It's a spiritual journey for him to collect the [Infinity] Stones and one with which he does not need armor. Once he starts acquiring the Stones he doesn't require armor the same way he did when he was a warlord. It's a very symbolic costume," he said.
Repeating my comment as a proper answer:
In addition to Ankit Sharma's answer, there's also a potential Doylist reason: it lets people see his face and expressions more, at least in regards to removing his helmet. Think of how Thor and Captain America don't wear helmets anymore, and how we always get inside shots of Iron Man's helmet to see Tony's face. (They haven't done it as much with Spider-Man due to the mask itself being expressive, I suppose. The same could apply to Deadpool, though the production for that is separate.)