21c is an Innovation Release so it can be used for testing of the new features. For Long Term Support, customers still now upgrading should be upgrading to 19c.
Update : I had published this blog post on 09-Dec-20 as soon as I saw Oracle's post that 21c was available. However, I now find that Oracle has updated the date of their post to 13-Jan-21.
I can understand web (blog) posts being updated but the actual publication date itself was changed by Oracle !
You can see Mike Dietriche's post dated 08-Dec-20 Even comments on Oracle's post supposedly of 13-Jan-21 are dated from 08-Dec-20 ! So, it is evident that the release of 21c was announced on 08-Dec-20.