Blizzard is known for giving full access to raid encounters on the PTR to competitive raiding guilds before a patch goes live, and while there is a limited testing period, this is enough for guilds to see the basics of the fight and begin coming up with strategies over the next weeks and months before the official release.
For the first time, Blizzard is considering not testing "final bosses" of the Sepulcher of the First Ones, which is not necessary to hide the encounter from world first guilds, but mainly to preserve the two storylines being told in the raid.
The Sepulcher raid will have a unique release schedule, where Anduin will be the 8th boss and end the first week of normal/heroic difficulty, and the final 3 bosses will not open until the following week alongside 11/11 mythic.
Lead Game Designer Morgan Day confirmed this in an episode of the Starting Zone.
Blizzard May Not Test The Final Bosses Of The Sepulcher Of The First Ones Raid
This is still an idea the team is figuring out and it will all come down to whether the team can release the boss bug-free despite no outside testing.
It is also not confirmed what the "final bosses" mean. Anduin has been described as difficult as a final boss and his story definitely agrees with it. The other option is the final three bosses including the final fight with the Jailer may not be tested, or only Anduin and the Jailer will be excluded.
We will know more once the PTR is out in the coming weeks and we get closer to scheduled raid testing nearing the end of the PTR cycle.