RSS feed showing deleted post

Isn’t /feed/rss the rss1, while /feed the rss2 (the wp default)?

For the upgrade message, delete the .maintenance file in your site’s root dir, and do a manual upgrade if the automated one failed.

Re the deleted post, do a hard refresh. The expiry date of your feed might be erroneous or something like that, and a proxy or your system might be using a cached version of your feed indeed.