Most profanities are allowed on Showdown. None are censored by default, and the owners are under no obligation to censor them either. As things go with much of the internet, if you don't want to see harsh language, it's on you to avoid specific websites. Showdown used to be 13+ and those sentiments still linger, especially with the site's connection to Smogon.
With that said though, Showdown does have its own rules list that prohibits some off-colour behaviour. This relates mostly to sexual content, which from my (limited) understanding of Showdown's administration, I know is dealt with quite aggressively. But outside of that, I don't think harsh language will be acted upon unless it's used harmfully.