Depends on how you say it.The real question is whether the factuality of a mythological narrative that may be part of someone's religion (and that's effectively all of them) can be questioned. If you talk about a global flood referenced in a mythological narrative, am I allowed to point out that according to archaeologists and geologists, that didn't actually happen?
As noted, I’m a practicing Roman Catholic. Generally, my brand of christianity doesn’t advocate biblical literalism. So I have no problem discussing parts of the Bible as being mere fables, told to make a theological or ethical point.
Heck, I even took classes in my college’s Religion department that treated all faith traditions as equal, and a course in the English department that taught the Bible as a literary work, and how authors have used its themes in their own work.
But, it should be noted, that some people do.
So, pointing out that flood narratives are common to a number of faith traditions is cool. Mentioning that there’s no scientific evidence to support any of them is OK. If someone posts a link that purports to show evidence of an antediluvian flood, you’re on safe ground if you can refute it without getting personal or otherwise uncivil.
But if you start spinning off with insults to the poster or adherents of the faith, calling a faith tradition “fraudulent”, using popular internet put-downs for the faith being discussed? THAT will get you unwanted attention.
However, as Umbran noted, before doing ANY of that, ask yourself why you’d need to. If we’re talking about. RPG settings, what the point of noting the absence of evidence for an antediluvian flood? If someone posts a link claiming proof, what’s safer- refuting it or reporting it for drifting into the religious weeds?