On April 27th, 2018 the Steam distribution was updated, changing the games executable from version 1.7.568 to 1.6.13620. While this was technically a downgrade, this was a new 1.6 revision making it incompatible with any remaining 1.6 servers, and certainly not 1.7 servers. Making things immensely more complicated, Activision pushed another update on May 3rd, 2018. Again changing nothing but the executable, this time to an official 1.8 revision (1.8.13620) (not to be confused with the "CoD4X 1.8" client that's been floating around for the last couple years which you should avoid like the plague). Just as before, this update makes the client incompatible with 1.7 servers.
This was done seemingly as a direct response to this Reddit thread where some users oh-so-helpfully seem to have gotten Activisions attention regarding an ancient and well-known exploit. A point of note here is that the exploit Activision "patched" only effects peer-to-peer games (CoD6-CoD69) not server based games such as CoD4(PC). Whatever else may have been done to the executable is unknown at the time of writing. Not that it particularly matters. The Steam install base represents an incredibly small fraction of the player-base, and with that being the case 1.7 is going nowhere.
The fix is relatively simple. download the 1.7 .exe linked below and use it to replace the iw3mp.exe in your install folder. Unfortunately Steam has removed the ability to completely stop a game from receiving updates. However, CoD4 doesn't require Steam to play, so you can simply move the "Call of Duty 4" folder out of Steam's directory and put it literally anywhere else. If you don't want to do this, at least make a backup of the folder, or at the very least the 1.7 executable as there's nothing at all stopping another "patch" from doing this all over again.
Zip password is: lel