I have a domain where 1 odd 1 letter sub domain is still indexed and the URL is:

m.domainname.com

The main domain has been deindexed in the latest wave and it seems the indexed sub domain page belongs to the original site that was hosted before I purchased this domain.

You think there's any chance to rescue this site as it isn't fully deindexed?

I also have some other domains where the same type of thing happened, all pages deindexed besides one stone old page but then without subdomain but more like:

domainname.com/old-page.html

No idea what to do with these and no idea why Google decides to keep showing one old page. Perhaps I should rebuild it to it's original state to see what happens.