Where is a list of breaking changes published for each new version?
CiviCRM does not have a "Known Issues" report per se, and I do not believe I have ever seen such a section in the announcement of a new release. What it has is JIRA (issues.civicrm.org), which provides many tools for tracking problems great and small. 

This is where you report and track specific bugs; for example, CRM-17968 scheduled emails not processed is reported to be fixed as of 4.7.4. But you are also given many tools for navigating the issues according to severity, resolution progress, and anticipated correction. You can see planned releases, the Change Log of which fixes were applied to which versions, and recent progress.

Broken functionality is not always detected during pre-release testing. I think if the core development team knew critical functionality was broken, they would fix it rather than include it in a report. Helping test the beta versions, either on your system or on demo.civicrm.org, will help prevent the software from going to release with major unresolved problems.

Related: What should I do if an issue is marked as fixed in JIRA but it is still occurring?



Obligatory lecture: the installation and upgrade guide, for one, emphasizes that you should back up your databases before attempting an upgrade. Your organization should have its own change control protocols for releasing new software to make sure that critical functionality does not suffer, and the system should be tested thoroughly before moving it into production. Your inability to roll back to 4.5.6 is not the fault of the developers.