Despite its client-side only effects, using the mod will render you unable to play alongside Vanilla players.

[SplachiFrenchSquid]

Title is self-explanatory, cannot join Vanilla players lobby while using the mod while it's effects are purely client-sided, which a bunch of other mods have no problem with.
Using a function similar LC_API's "EnableNetworking" option could be a potential way to adress this issue.

[AinaVT]

Are there any logs? I just tried joining a vanilla lobby with only LethalConfig on the second client and was able to join normally.

[AinaVT]

Also, a thunderstore/r2modman profile code would be appreciated.

[FerusGrim]

This is inaccurate? I use LethalConfig and a plethora of other mods that are client-only and can join vanilla lobbies, even host vanilla lobbies, with no issues.

Surely you've got a server-side mod you're not thinking of? Have you tested with JUST LethalConfig to see if you can join vanilla lobbies?