Although I like the idea, there's no reason to posit the preservation of the myth of a lost star for tens of thousands of years. People can still see all Seven Sisters under the right conditions, and the right conditions were probably much more prevalent back before the insane degree of light and air pollution that exists today.
When I was young, I was told a story about how native tribes used the star as a vision test for warriors. Only people with excellent vision could discern the second star.
No idea if there's any truth in the story, but I wouldn't be surprised
You're probably thinking of Mizar and Alcor, which are at the middle of the handle of the Big Dipper. This was used as a vision test in ancient times and in the middle ages.
A fun fact is that it is actually six stars. Alcor is a binary star system and Mizar is a quadruple star system.
https://skyandtelescope.org/astronomy-news/many-pleiades-can...