To reproduce:

Ping a forge server in the multiplayer menu that does not have Clumps in its mods folder. The client will display a red "x" for incompatible even though you are able to join the server. Is this intended? Because I assumed CLumps is okay to run just client-side.

Versions:

Forge 1.16.5-36,2,6
Clumps 6.0.0.26

From clients latest.log:

[02Oct2021 22:17:38.148] [Netty Client IO #3/INFO] [net.minecraftforge.fml.client.ClientHooks/CLIENTHOOKS]: Client has mods that are missing on server: [clumps]

In 1.16, Clumps is required on both sides.
In 1.17 it is only required on the server.

So

Because I assumed CLumps is okay to run just client-side.
Is a bad assumption.