Servers get labeled as incompatible when the server is not running Placebo. I don't think this is intended because I'm pretty sure that Placebo is meant to be a client-side mod.

from client latest.log
[20May2021 16:30:50.130] [Netty Client IO #7/INFO] [net.minecraftforge.fml.client.ClientHooks/CLIENTHOOKS]: Client has mods that are missing on server: [placebo]