It's an endless arms race. I spend a lot of time studying unsavory people and a great deal of effort goes into the development of new dogwhistles that are designed to either provoke or connect with peers while maintaining deniability. You might like this paper on the evolutionary dynamics of covert social signaling: https://www.nature.com/articles/s41598-018-22926-1
I can really identify with that. My efforts are against spam and trolls; at least the spam is predictable and easier to take a sledgehammer to! Those grey-area trolls are such a miserable part of online content and moderation.
If they're covert and only members of the group using them know what they mean, then why fight it? Seems like a helpful way to communicate about things that others might not like to hear. The paper you linked said that too - "Such signals may allow coordination and enhanced cooperation while also avoiding the alienation or hostile reactions of individuals with different preferences.".
This happens all the time in cartoons that appeal to children but also contain subtle adult jokes so everyone can enjoy them on different levels.
Because it encourages more of the same, and it's rarely completely covert. Often and increasingly, it's racial dogwhistling and just devolves every thread to wasteful, antagonistic and unproductive conversation. You might've heard the line/story about accommodating nazis in your bar.
