I apologize in advance for the general question! Perhaps this might not even belong here, it might be more of a sociology question. I am not sure but I hope someone here can shed some light.
Time: Before writing became commonplace
Place: I am going to assume that would be not as relevant as I expect the same principle to apply more or less universally. I hope that is okay.
Clarification: I am aware that over the many years oral traditions would change and alter but today I heard a claim (in the context of mythology and meta-stories) that over time, orally passed-down stories, specifically myth stories, were stripped down to their essense and any irrelevant details were dropped, leaving behind only the details which had a meaning. This seems to be counter-intuitive as I would have expected the exact opposite to happen: over time stories would become more and more embellished with each generation adding something to the story. Is it possible to make a general claim (i know, terrible) about the validity of this?
I am aware there will be scant, if any, evidence for the gradual progression of oral stories but perhaps we can draw some conclusions (gasp!). Google has not been very helpful -unsurprisingly-I really hope someone on here can at least point me in the right direction. Much appreciated!
The subject you raise is at the heart of classic studies of folklore, which considered how folk narratives diffused and changed over space and time. Volumes have been written on this, so I offer only a brief synthesis of what has been observed and discussed.
There is a duel process of deletion and augmentation that affects of living oral narrative. If a storyteller tells a story with a motif that simply does not work (because it conflicts with the local culture or it is simply bad storytelling) that motif will be dropped by the smoothing process of repetition. At the same time, storytellers would embellish, and some of these additions would survive the processes of repetition.
Things were/are complicated by diffusion over space and time. As cultures change over time - conversion to Christianity or the effects of early industrialization - this caused a shift in the nature of oral tradition. The same thing happened when oral narratives diffused and were adopted by cultures that might not understand certain motifs. In both cases (time and space), some motifs were no longer appropriate and were trimmed away, but invariably, new ones were introduced. The effect of these changes was not to trim down oral narratives but rather to transform them. They did not become sparse because of the transition but they did change.
/u/Itsallfolklore has already given extensive and excellent answers about folklore, so I want to approach this question from another angle.
Oral traditions are very important sources of information about African societies prior to colonialism. The Belgian-American historian Jan Vansina was an early champion of treating oral traditions of culture-heroes and of kings lists as historical testimony. His book Oral Tradition as History deals extensively in the theory and praxis of working with these traditions.
In that book, Vansina identifies several cliches that are common to the culture-hero myths among many societies in Central Africa.
For example, many culture-heroes are often skilled hunters who leave the lands of their birth and as young adults found kingdoms of their own. These culture heroes are often credited with bringing social innovations (like kingship) or technological innovations (like iron working, or new plant crops) which provides wealth for the kingdom.
Vansina also observes that while the founder is often remembered as a hunter and warrior who established and expanded the kingdom, very often the king-lists remember his successor as an organizer, administrator, and lawgiver.
He observes that in recitations of anecdotes about pre-colonial Rwandan kings, there is a cyclical theme that A is a founder, B is unlucky, C prospers and D is a lawgiver.
Vansina ascribes these tendencies to a process of "structuring" where events, personalities, innovations, causes and effects are edited and re-organized according to the worldview and collective memory of a community. That is, this structuring is the crafting of a memorable narrative.
Vansina also argues that an effect is that oral traditions of king-lists tend to have an "hourglass shape". Meaning, events and innovations are attributed to the distant past, in the early generations of a dynasty; while events still within living memory or in recent lifetimes are remembered in the narrative. But between the distant past and the present, narrative becomes sparse.
So, in those ways, what Vansina is saying appears to conform with the claim that you heard.