it used to, not anymore. there is now a distinction between archival relays and non-archival relays. Archival relays indeed store the entire network history, but it turns out that this is a very different purpose from the relaying of data from pds to appviews
for the network to function you simply need the firehose, and that turns out to be really cheap
building an offsite archive of the entire network also has its uses, but its not related to the function of the relay
「ソーシャルメディアとしての」というか、より厳密にはリプライなどの通知を伴うようなコミュニケーションの機能とでもいうべきか。特定のユーザの投稿を購読するだけならPDSを直接叩き続ければ良いだけなので