I believe you're asking the wrong question for what you mean.
Myths and mythology are broader terms than just stories from old, largely historic religions. There are plenty of contemporary myths that have nothing to do with religions.
I think, and feel free to correct me, what you're really trying to get at is
Should ancient/historic/largely unfollowed religions be on topic?
Your contention is that since no one follows them anymore, they are no longer "religions", and have crossed over into some sort of public domain, for a lack of a better term. In a sense, this is true. The average person doesn't think of Zeus and wonder if the guy next door secretly has a shrine to him. For many today, Greek or Egyptian "myths" hold as much truth as Grimm's Fairy Tales.
Without trying or wanting to get into a big religious debate, I'm going to put this out there: to me, all religions (current, or ancient, and regardless of the numbers of followers are equally true/untrue/legitimate/illegitimate as none of them can be proven as more or less true than any other. This is why I mentioned in comments that defining a "myth" based on the definition you provided is such a dangerous idea. If we hold that any religion is false, we must hold them all false, and if we hold any of them true, we must hold them all true. That's not a discussion we want to be having.
We have already banned religious texts such as the Bible and no one has ever contested that - only the scope of how much we were going to ban. Now, to my knowledge, there are no Ancient Greek Holy Texts (I could be wrong; I'm no expert on the subject), but religious teachings, beliefs, rules, and dogma stem from more than just texts.
That said, as discussed here, allowances are made for fictional works based on the religions/myths, provided that they are intended as a fictional work, and not a documentary.
Questions about the mythology mentioned in Clash of the Titans? We're good. Questions about the mythology mentioned in The Mummy? We're good. Questions about Noah, the movie? We're good. Questions about The Iliad or The Odyssey? Maybe a little fuzzier, but I'd say we're good. But questions about religions in general? I'm going to say no. We may have a lot of overlapping interests, but we (as a group) are sci-fi/fantasy experts, not religious experts.