On Monday night the AP reported the particulars of that provision:
The policy, obtained by The Associated Press, prohibits "requiring, coercing or encouraging" players from "dressing up as women or wearing costumes that may be offensive to individuals based on their race, sex, nationality, age, sexual orientation, gender identify or other characteristic."
The story notes that it's unclear when this practice began. I know it wasn't mentioned in Jim Bouton's "Ball Four" or in either of Jim Brosnan's superb season diary books, "The Long Season" (about the 1959 season) or "Pennant Race" (1961). Bill Freehan's diary book about 1969, "Behind the Mask," tells of a prank pulled on the Tigers' rookies of enticing them to go to a marina for a team boatride that didn't exist, but nothing like today's dress-up.
But the story tells of Jeff Kent's rebellion in 1992, arguing that he had put up with it the year before and wasn't going to accept it again. So it was happening by the 1990s.
Personally, I find hazing a profoundly silly ritual at best and often damaging. This is true whether it's involving a marching band, a fraternity or a professional sports team. It tends to escalate, and what seems like harmless fun turns over time into something uglier.
I'm sure there are players disappointed that their bullying ritual has been prohibited. I'm also sure there are players relieved by the ban, which is, as the story details, only partial. There are some costumes still permitted.