Oracle APEX 21.1.0 was released on May 12, 2021.
Here is the shortcut link to the download: apex.oracle.com/download/ and don’t forget to apply the Patch Set Bundle for Oracle APEX 21.1

As you already know from my previous comparison posts (e.g. this one), I always start by looking at the changes on APEX View’s and API’s. This will reveal all the important fundamental changes.
Of course we won’t catch UI changes this way, but most features are based on Metadata/View/API changes, so this approach will give us a good overview of changes.