When removing a status effect client side, TslatEntityStatus tries to do a sync.
However this uses server side code on the client, which causes a crash.
The issue seems to come from this mixin: https://github.com/Tslat/TslatEntityStatus/blob/1.21/Common/src/main/java/net/tslat/tes/mixin/common/LivingEntityMixin.java

See this issue for a crash report: Sjouwer/gamma-utils#61

For a little bit of context regarding Gamma Utils:
Gamma Utils uses client side status effects to provide an indication of when a user has extended Gamma or Night Vision enabled.
This is disabled by default, but can be enabled inside the config settings.
When enabling this together with TslatEntityStatus it'll crash the game as soon as the status effect gets removed.
See the linked issue above for more info.

What should and shouldn't be done on the client is debatable imo. The code I use however is purely client side. It's also done purely to have a visual that feels familiar and fits well into the game, it doesn't do anything else.

=/ you shouldn't be doing that on the client side lol

@Tslat So can you fix this, please? Both mods are pretty popular so I'm sure plenty of other people faced this same issue, it would be great if you could solve this incompatibility, and from what I understand it can't really be solved on Gamma's end since a feature depends on it.