A server transmits its mod list by sending chunks of mod information using A2S_RULES. Each mod in the mod list consists of an ID represented by a 32-bit unsigned integer and a name represented by a variable-length, null-terminated string. These IDs are just the mod's workshop file ID. These however were intended by Steam to be 64-bit unsigned integers. Between my workshop items 2990814733 and 3260710486 lay about 352.069 days - roughly a year. Within this time frame, about 766598 workshop IDs were created per day. If this trend continues, the last representable workshop ID of 2^32 = 4294967296 will be surpassed in around 3.7 years.

This means that any mods created after 3.7 years from now, will by default be incompatible with servers. Client mods should still work, but new server mods will have their IDs truncated - no launcher would be able to get the correct mod ID and joining a server with such a mod would become impossible. Please fix :)