Hello!
The forum I help moderate is currently ‘testing’ if a profanity filter would be useful.
At the moment, moderators are being notified about the filter picking up watched words in private messages (the entire message being unnecessarily visible).
Is the watched words filter a ‘catch all’ filter, or is there a way to exclude the private messages.
Thank You! 
Ficus
Hello there @Ficus1 welcome to the Discourse meta community 
!
This is more of a specific site issue / feature. In the whole scheme of things, seeing the whole flagged message is a good idea because there might be something else that wasn’t flagged that is inappropriate etc. In personal messages some users seem to get an illusion that they are behind closed doors and so think it grants them a pass to be nasty about other people behind their back. This is why (and for other reasons) why the watched words filter expands into personal messages.
thank you @ondrej. We will have to consider this as we move forward with using the filter