You can implement a server without all those tricky features. Proxies are hard and a design wart if you use them. Quoting your link it describes this politely,

You can implement a server without all those tricky features. Proxies are hard and a design wart if you use them. Quoting your link it describes this politely, "By itself, this is harmless. However, modern websites are composed of chains of systems, all talking over HTTP. This multi-tiered architecture takes HTTP requests from multiple different users and routes them over a single TCP/TLS connection [...]"