I've seen it brought up in some answers on here that indigenous oral history, more specifically Amerindian oral history, has been remarkably resistant to pure legend and "games of telephone" compared to, for example, European written records. An example of this is, if not Amerindian, if I'm remembering it correctly, is that in Australia there are aboriginal peoples that tell stories that of some sort of calamity, probably a vulcanic eruption, near a specific body of water, which when compared to what evidence we can gather, seems to indicate that through thousands of years these people were able to keep the memory of this event alive throughout a period stretching thousands of years. I just want to know how this is possible? Is it actually a thing that oral history is able to stay so accurate over such a long time?
Although it's possible that it's one of my posts that you are referring to, here's a couple posts I've made on the subject:
How do historians conduct research on oral history?
And another story you might be thinking of would be this one about aboriginal Australians having stories of sea level change going back approximately 10,000 years. link
As to keeping something alive - if you look in to urban legends, for example, and try and track them down, you will go through hundreds of tellings, thousands in fact, likely without ever getting back to the origin. Oral histories, over the course of several thousand years, actually rely on fewer tellings, I think our reluctance to give them credence is linked to a lot of factors in our lives today, such as an overabundance of stories, to where everyone can have completely separate interests, or the massive reduction in time we now spend sharing stories with each other compared to in previous generations. Many of these oral histories have concrete memorials, such as an ocean, or an old lava flow, or a large flood plain, and so on, while others have been memorialized by naming mountains or rock features after them. All this makes passing on a story like this fairly easy, and also immediately useful - it becomes a part of knowing the landscape, just like we learn street names. There are a lot of other reasons as well, but these links are a start.