It should be checked from client side or else the server would be easily overloaded. It really depends on their architecture. The process of update filter words can actually be done in real time. No need for maintenance at all. But it may use a lot of bandwidth if it frequently sends new information to all clients. I know because I used to work with such system.
As I’ve already said no method will 100% work. It must use several method together. You should see what IMC currently do. They try to detect bot by using Machine learning with Heuristic function. Can it detect all bot? No. They try to restrict trade in game to prevent RMT. Can it prevent all RMT? No. So, just because it doesn’t always work. Should they stop what they are doing? … Based on my current experience, the best method to prevent spam bot is to close server from new account. I’m in Telsiai and I’m very happy that there is no more spam bot in town. Server performance is also better. It’s definitely the best method but is it a good method? Nope
Just saying that this does not work is also very easy to say. Don’t forget that you can see the flaw of detection method because you know how it is working. Hackers/Exploiters will always find a work around method if they know how the system work. That’s the reason why the detection method must not be disclosed to public. My method is just an easy fix. Of course, it cannot cover to all cases. It can actually develop further to process a more complex logic. There are many research about chat bot. If they limit the length of chat message then it shouldn’t take long to process each string text.
Edit: Also, the easiest solution to fix those characters is to limit character in font. Main language for this game is English. There is no need to support all language and all symbol. So, it can reduce many characters to small amount.