An update to the MultiVersus website has supported the rumour that Rick will curse in his dialogue.
One of the longest-standing rumours and worst-kept secrets is that Rick Sanchez was eventually going to be a part of MultiVersus, having been leaked alongside the game late last year. That rumour was finally confirmed yesterday, with a reveal at San Diego Comic-Con showing LeBron James, Rick, and Morty are the next three characters coming to the game, with LeBron coming next week and Morty and Rick coming during season one.
Following the reveal of those three characters, the MultiVersus website was updated to include them on the roster. Usually these character pages aren't too interesting and just give a bit of background information on the fighter, as well as their class and special moves. However, Rick's is particularly interesting here as it seems to support the rumour that he'll be cursing in his dialogue.
Rick's page has a quote from him at the top that reads, "Let me day ex machina this sh*t and go home!". Ignoring the incorrect spelling of deus ex, the fact that Rick is openly cursing in his quote, albeit it slightly censored, implies that he'll also be able to do the same in-game.
When Rick was first rumoured to be coming to MultiVersus, the same leaker who revealed him said that he'll be cursing freely and that players will have an optional mature language filter to block him out. This was backed up by the game's ESRB rating, which listed language as one of its content indicators.
Players currently playing MultiVersus have noticed a mature language filter in the options menu, which matches up with that earlier leak. The current version of the game has a few characters like Harley Quinn saying words like "crap", but seemingly nothing so bad to need a content warning, implying that its use is for fighters like Rick that will apparently be swearing as normal.