This is put in logs when using something that uses Multiconnect API:

[main/WARN]: WARNING! Mod multiconnect-api is only using deprecated 'modmenu:clientsideOnly' custom value! This is no longer needed and will be removed in 1.18 snapshots.

[main/WARN]: WARNING! Mod multiconnect-api is only using deprecated 'modmenu:api' custom value! This will be removed in 1.18 snapshots, so ask the author of this mod to support the new API.

Fixed by #164