Agreed. Many modern systems need only TLS 1.3 and nothing older. Unfortunately, TLS 1.3 is saddled with the baggage of backward compatibility. This is not something you might care if you're deploying it, but if you're studying how things work, it's still necessary to understand TLS 1.2 and earlier protocols. There are many places where, in the TLS 1.3 chapter, I had to say something along the lines "this design decision ties to this TLS 1.2 behaviour or problem".