I just came back from 29 Apr cruise with a 2.30pm boarding time, and I can safely say that I was able to book the shows and activities in the Royal app, such as Sequins and Feathers and Viktoria, as well as Bumper Cars etc with no issue within the first hour of boarding while eating at Windjammer for a late quick lunch at the same time, so not sure why suddenly have this only can book after 6pm rule?
You do have to act fast though because by around 4pm to 5pm plus, most of the slots are already taken, so best to go down to Guest Services first and get the Cruise Compass before booking what you want.