Hey cootje,
On Realms, Mojang reads every single chat message and is extremely trigger-happy with banning players.
For example:
Here is a Swedish player who got banned for saying "slut", a Swedish word meaning "end", on their own Realm:
Here is one of many examples where a group of friends were just casually swearing around each other, on a private Realm, and got banned:
And here is someone who got banned for insulting an enderman on their Realm:
I would absolutely recommend against using the chat on Realms, regardless of whether or not you have Wurst or any other mods installed.
On normal servers, however, the chat is far safer. There, Mojang can't see your chat messages unless one of your friends reports you through the in-game menu. Even then, there are server plugins you can install to prevent the report menu from working.
If you are the owner of the Realm, I would recommend that you cancel your subscription and get a normal server instead. It will almost certainly be cheaper, you get more features like the ability to install plugins, and you won't run the risk of getting globally banned from Minecraft for saying "fuck that creeper". 
Hope that clarifies the risks of chat reporting! Let me know if you have any further questions.
