I'm looking at this new question What are the components of a legend (in the sense of a tale, not a figure legend)? and went to add a mythology tag, but we don't have one!
As the question I linked to points out, creating such background for your world is often an important task. This question is on topic because it's asking how to write the legends. But other questions about mythology and the like are also on topic for Writing. An author can use myths for research, inspiration, or character development. Or an author may wish to duplicate or echo the structure of a myth. In addition to the story in the above question, many authors write folktakes and such either for their world or just outright. It's a genre, and not one you can just toss into fantasy
There are 26 questions with the keyword "mythology" plus another 14 (non-dups) with the keyword myth. Only 2 questions with "folklore." And 1 question with "folk" that does not also have "mythology" and is about folk tales. 1 question about folktales. 22 questions about legends.
I have not checked all these for duplicates or suitability for such a tag, but it looks like there are a fair number.
My suggestion is that I start a tag that encompasses all these things. The primary name can be either mythology or folklore or folktale or legend, but we might want to consider synonyms too. The reality is that you can not search the text of tags, only the names (and the synonyms don't come up under tags, only if you're adding to a question).
Note: that this is a related Meta discussion, though not a duplicate. Do we need an 'ethnicity-religion-race' tag?
Please let me know what you think.