Proving Grounds Nightfall went live in Destiny 2 with the weekly reset on March 30, 2021, but if you were hoping to play it and now can't do so, it's because Bungie retracted their previous statement.
When Bungie talked about the Season of the Chosen roadmap initially, they announced the Proving Grounds strike as exclusive to Season Pass holders for the first week but noted that the strike would become available to everyone afterwards, along with the Nightfall version of it.
Well, that will not be the case as they announced that this particular Nightfall will be exclusive to those who own Beyond Light and Season of the Chosen Season Pass, despite the fact that it's set on Nessus, which existed long before all this Stasis mess was introduced.
The announcement itself was brief and only stated that the official roadmap calendar will be updated to reflect the new stance. Naturally, this didn't go down well with the community, who expected to jump into the Nightfall with their friends.
That said, there are currently no indications that Bungie will be reverting back to what was originally promised and will force players to buy the expansion and the pass if they want the privilege of tediously farming for masterworking materials, Exotics and Adept weapons, none of which are guaranteed to drop at the end of the run.
If that wasn't enough of a push to buy Beyond Light and the pass, Nightfalls will be offering twice the rewards this week.