The job listing was published in October 2020 and didn't catch press attention until this week. Once the post started circulating on tech sites, Microsoft edited it to cut out the interesting bits.
WindowsCentral's Zac Bowden published a piece on Sun Valley in October, with information mysteriously attributed to "sources." ZDNet's Mary Jo Foley followed up to say that her unnamed contacts confirmed that the project exists—and she has seen engineering references to a "Windows 10++" due next fall—but Microsoft's official response was a rather cold lack of confirmation:
It's not new for Microsoft to deliver some Windows features through cumulative updates. We have nothing further to share.