Destiny 2's Prestige raid was supposed to be already available today for players looking for some higher difficulty pleasures, but the content deployment will be delayed due to exploits that have become apparent during testing.
Destiny 2's Prestige raids were envisioned as a trial for players looking for an extra bit of challenge on top. The initial idea wasn't to make these raids eternally grindable, but rather provide dedicated players with some unique bling for completing the challenge. This purpose seems to have been somewhat undermined in initial testing runs, as players have found creative ways to complete the raid, in other words - exploits.
"An exploit has been discovered in the Leviathan Raid that causes the encounters to be stripped of their intended challenge. To allow time to fix this issue, we are delaying the start of the Prestige Raid until next week. It’s important to us that the team who earns World First status doesn't have their legacy tarnished by doubt, scepticism, or uncertainty", was the official explanation for the delay posted on Bungie's forums.
The raid has been postponed for 18 October 2017. The raid won't have the usual gear based rewards, but will instead allow players that manage to complete it to "shine" - which probably means cosmetics of unique titles.
Concernes over the exploit seem quite justified in this light, especially since a fairly small number of players will be able to claim the "World First" status, mentioned in the announcement.
"Our goal is not to provide a boon to your character progression, but there are unique rewards to help you shine. Consider this your invitation to prove to the world that you are among the very best of the Raiders who overlook the City", is the way Bungie explained their idea for Prestige raids.