Can someone give me more of a hint of that reasoning so that I can at least search for it? I'm intrigued, but the search terms I'm trying all come back with explanations of why you might need Content-Length, a different issue.
HTTP/2 allows you to avoid that by saying "I'm done with this stream now, sorry!"
H2 let's you just send the server a short message that says "stop sending that stream" and you can go ahead and pipeline a new request right along with that cancel.
This happens a lot more than you think as you browse through a collection of things and are just scanning them and clicking the next button - that's a really common use case h2 will handle much better.
So how most web applications handle request bodies that are too big is to either kill the connection (which to the client looks like a network issue) or patiently pipe the entire message to /dev/null before sending a response that indicates the message has not been processed (e.g. HTTP 413).
Unless I'm mistaken, this can be avoided if the client sends an honest Content-Length header, but this only works if the size is known in advance and the client is honest (you could submit an arbitrary number of bytes with a Content-Length header indicating something much smaller).
Because there is no clear way to distinguish a server aborting a request mid-body because of the size versus spontaneous existence failure, a client might misinterpret that and attempt to re-submit the same request later.