looks like 7.3 is the latest version of C# for the 4.7.2 Framework

Wikipedia has this cheatsheet that I stumbled onto today:

It touches on the confusing history of the split between .NET Framework 3.0 and .NET Framework 3.5 (the later introduced LINQ and Lambda stuff) even though there was no C# 3.5 and C# 3.0 straddles them both. The LINQ and Lambda stuff used compiler tricks to compile to the same CLI code the 3.0 framework did. I guess the LINQ and Lambda stuff just got looped in via some new libraries. I have heard the big difference between 1.0 and 1.1 was the using statements.