To add to this, C# added async/await in 2012. Depending on what frameworks you use it may be hard to avoid interacting with it (for example, many APIs in UWP are async without a sync version). I feel like other language features added since then you can get away without knowing while writing code, but of course you may encounter them when reading other’s code.

That being said, C# 5 code can be retrofitted with most new features simply by listening to the IDE which suggests refactorings to make use of them.