I am looking for a definitive list of Microsoft.Exchange.WebServices.dll versions that are supported and those, if any, that are EOL. Anyone know for sure?

Regards

What type of supportability are your after ? the Managed API is just a client side library older versions of less functionality (access to newer EWS operations in new version of Exchange) and don't allow you to specify the latest request server versions (you need to understand Schema version for that to make sense https://msdn.microsoft.com/en-us/library/office/dn741586%28v=exchg.150%29.aspx ) .The Managed API is now open source https://github.com/OfficeDev/ews-managed-api so any bugs you find you can fix yourself and its backwards compatible so the latest version will work with 2007 up. With EWS the supportability is generally more concerted with the Server side so if your still running Exchange 2007 or 2010 these versions are  is now going into extended support which will have an effect on the support experience you will get if you experience a new server side bug in EWS have a read of http://blogs.technet.com/b/mr_proactive/archive/2012/05/10/support-lifecycle-and-products-you-need-to-consider-upgrading.aspx .  

Cheers
Glen