We have converted Left 4 Dead 2 to be delivered through SteamPipe.
Next Monday (July 1) servers downloaded with the HLDSUpdateTool will no longer be compatible. Only the server obtained using SteamCMD will work. The server is currently available for download; we encourage all server operators to download and install the server now if you haven't already. The server is interoperable with the current public Left 4 Dead 2 client.
The AppID for the SteamPipe version of the server is 222860.
Instructions for using SteamCMD to update your servers are available here:
https://developer.valvesoftware.com/wiki/SteamCMD