How to check if a 2013 or 2016 Sharepoint site collection is backward compatible (lower compatibility level 12 14 & 15 )
SharePoint isn't an application that have the features of backward compatibility. Often the term backward compatibility refers to client applications such as Word or Visual Studio. That means that a document created in Word 2016 can be used by Word 2010, but not Word 2007. Word 2016 documents are compatible with Word 2010, but not Word 2007.
In SharePoint, which is a server applications, backward compatible does not apply. But you could take a non customized page created in SharePoint 2016 and use it SharePoint 2013 or SharePoint Online. But this doesn't work on SharePoint 2010. So in a sense one could say that SharePoint 2016 is backward compatible with SharePoint 2013.