To be very clear/direct, the HTTP pipelining you're referring to never fully worked and was always hit with problems with middleboxes, problems with servers, si

To be very clear/direct, the HTTP pipelining you're referring to never fully worked and was always hit with problems with middleboxes, problems with servers, significant security vulnerabilities around request smuggling and more besides. Every project I've been involved with that tried to use them eventually turned them off. So the quote is true in practise.