Just reading the Doc 742060.1, the release 20c, which was due 2020, disappeared from the roadmap. Don’t wait anymore for 20c, there won’t be one. There was a preview release in the cloud, but Oracle failed to release one new release every year. While we are all used to wait 2-6 years for a new major, switching to yearly versions (18 and 19 were just patchset) is a promise Oracle couldn’t hold.
My two cents : desupporting non-cdb in (invisible) 20c is an headache for many customers…