Some shameless Overwatch players have been using the new custom game server browser to farm for XP. Custom games reward experience upon completion, so players were setting up Skirmish matches in which they could safely AFK, collecting experience without any effort. The practice was always ethically dubious, but most players assumed it existed in some sort of grey area as far as Blizzard was concerned.
Consider that grey area now officially black. In a forum post, game director Jeff Kaplan made it very clear that Blizzard does not approve of the practice.
Kaplan went on to explain that the Overwatch team expected this sort of thing to probably happen, and has an "escalation plan" in place to combat the behavior. The actions taken today—Skirmish mode in custom games ceasing to reward experience, and implementing an AFK timer for the same—are the first steps on that plan. If players don't shape up, the nuclear option means turning off experience gain in custom games altogether.