Just notice the suggested profanity filtering.
In general, the filter of profanity makes sense only if you are really confident in the native language of the target audience. Otherwise, you can make your international clients angry: there may be words in English that are profanity in another language (there is no way to find them all), and the words you filter out may be perfectly correct words in other languages. When the Sims 3 was released, EA forums filtered out the word “damit,” but they did it in localized forums, and “damit” is a very common German word. It made it difficult to write anything. “Sex” also means “six” in Swedish.
This may or may not be a problem if it is only about subdomains. I will just be careful and, rather, block words for technical and security reasons (or things like your own trademarks / products or something else).
It may be wise to use a special domain for custom things that do not have any subdomains, and use a different domain for any subdomains that you need. Thus, you will not need to block the subdomain names in the "user" domain at all.