First, I have to tell you that I have not been following Yahoo at all but I am assuming that the same thing is going on that has been going on for years with the Inktomi engine - which became Yahoo Search.
Inktomi conveniently developed a problem years ago - sites would mysteriously dissappear from their index on a regular basis. Their answer to that was to invent paid inclusion - a protection racket really - where you pay them to ensure your site stays in the index.
Yahoo bought all that and as far as I know the same thing is still happening. I check my sites and one day they are on top and next time I check they are gone from the index. A month later they are back on top.
Years ago I payed for inclusion to stop this but it resulted in lower rankings - the sites would stay in but rank lower. So, I stopped and my sites all rank well (when they are in the index).
Actually, things were stable for quite some time until recently so I did not worry about it. I assume Yahoo is just trying to coax more people to use their paid inclusion product so they started making things unstable again. I don't know if paying for inclusion still results in lower ranking but I for one am not going to waste the money to find out

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