What makes a block Moodle 2.3 compatible? (..if you compare it with 2.2 and 2.1)
To give an example: Quickmail is compatible with Moodle 2.2, so...
The convention now is that API changes are now documented in files called upgrade.txt scattered throughout the code-base. So, the relevant information is in http://git.moodle.org/gw?p=moodle.git;a=blob;f=blocks/upgrade.txt;h=b7e288e9c9bb2881fca82266547bc4ce5f1b07d9;hb=HEAD.
The key information like this should be summarised in the release notes.