Yesterday, Adsense support sent an email to their users regarding their upcoming policy changes. This primarily affects how subdomains are monetized. Going forward, your subdomains inside the primary domains in the "Sites" section (www, etc.) won't be allowed, any existing ones will be removed and their rules will be merged with the primary domain (such as example.com).

Furthermore, what constitutes a "Site" will also change henceforth. You can only add a primary domain (such as example.com) and the subdomains which are listed on the public suffix list (such as github.io, blogspot.com, etc.). Thus, your own subdomains (such as xyz.example.com or www.example.com) won't be allowed in Adsense.

I don't know what they will achieve by doing this considering they already vet and audit each site before approving them for adsense? In any case, other alternatives to Adsense exist such as Propeller Ads, CJ Affiliate, etc. for those affected by this move but I don't know their efficacy.