De acordo com a documentação:
Módulo Apache mod_proxy
This module implements a proxy/gateway for Apache. It implements proxying capability for AJP13 (Apache JServe Protocol version 1.3), FTP, CONNECT (for SSL), HTTP/0.9, HTTP/1.0, and HTTP/1.1. The module can be configured to connect to other proxy modules for these and other protocols.
Apache's proxy features are divided into several modules in addition to mod_proxy: mod_proxy_http, mod_proxy_ftp, mod_proxy_ajp, mod_proxy_balancer, and mod_proxy_connect. Thus, if you want to use one or more of the particular proxy functions, load mod_proxy and the appropriate module(s) into the server (either statically at compile-time or dynamically via the LoadModule directive).
Módulo Apache mod_proxy_http
This module requires the service of mod_proxy. It provides the features used for proxying HTTP and HTTPS requests. mod_proxy_http supports HTTP/0.9, HTTP/1.0 and HTTP/1.1. It does not provide any caching abilities. If you want to set up a caching proxy, you might want to use the additional service of the mod_cache module.
Thus, in order to get the ability of handling HTTP proxy requests, mod_proxy and mod_proxy_http have to be present in the server.
Em resumo, o mod_proxy_http pode ser usado apenas para manipular proxy HTTP e HTTPS e isso requer o mod_proxy (fornecer recursos para outros módulos)