Every company is stack ranking and every good manager (and many bad ones) have their teams ranked in the event that they need to cut or have the opportunity to improve the team through training, promotion, etc. ("What would you do if I gave you four open positions?")

You have that guy who you're putting on plan because he's late with his deliverables, doesn't really 'get' the purpose of the department/group/company, and kvetches all the time about one thing or another? You've ranked him.

That woman you're trying to mentor as much as possible so you can promote her, maybe even to your peer level? Ranked.

There are people who are essential to an organization at different times. Keeping the lights on? I've cut everything but devops and maintenance, including myself. Sometimes you can save someone, but you can't transfer a problem.

It might only be temporary at Yahoo. If they have a lot of C players, they might be calculating that there won't be a lot of false positives in the early purges.

eBay also has stack ranking.