Basically Valve decided to break with the way SteamIDs are displayed. It's apparently called SteamID3.
They've already implemented this in TF2, silently without any official word what-so-ever. So other games might follow (I think it's likely).
Big discussion @ https://forums.alliedmods.net/showthread.php?t=246705
Example: from STEAM_0:1:48287056 to [U:1:96574113]
How to convert:
Old = STEAM_0:A:B
New = [U:1:B*2+A]
Example: STEAM_0:1:1234 -> [U:1:2469]
Source: https://forums.alliedmods.net/showthread.php?t=248632