Well, based on how Dawn was talking to May when they first met, it sounded like Ash and Brock told her about May off-screen. They told May and Max more about Misty off-screen as well. I know that Ash mentioned Misty once before she appeared, but it wasn't much about Misty herself and May did mention that she had heard a lot about her when Misty came back for those couple of episodes.
I didn't think that 4KiRAB kept on adding in Ash x Misty moments even after she left. I know that they added in that line in the Jirachi movie, but I don't remeraber anything else after that. Whenever Misty came back, Ash would just be treating her like a good friend and vice versa.