Just to add on: A more detailed link would be here

On another note, I doubt Msian players (read: those on flying spaghetti monster) would be playing this map anytime soon. Reason being, there's no incentive for them to even play this map. It does not contain any content changes, but merely a compatibility issue with 1.24 patch. We all know most Msian players are still playing with 1.21b versions in CC or their home PC. Hence they would still continue playing with 6.61b.

However, as IceFrog has mentioned, future map releases with content changes would definitely force players to update their versions to at least 1.24 to continue playing Dota maps. But until then, this map would disappoint many ignorant players.

p/s: 6.61c cant be played on version 1.21b.