Google Adsense is bringing a bunch of policy changes that affect how your sites are monetized
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.
I would guess this is to prevent the classic pattern of
good-site.com.bad-site.comfrom showing up as the source. Especially if it's truncated in results. Unfortunately we got the display order of domains wrong 30 years ago, and it's too late to fix it now.It's good that they're using the public suffix list though since that includes GitHub Pages and other common providers. It's a bit of a messy system, and goes against the whole decentralized notion of the web, but at least it works well enough.