Four days after saying it would not, Bulkhead opened the doors. A two-line post went up on the Steam news hub at 21:08 UTC on Saturday 5 September:
"THE CLOSED BETA IS NOW AN OPEN BETA! Come break our servers ⛓️💥 Until the Beta ends - Sunday at 08:00 UTC."
That is the whole announcement. No new end time, no new route in, no mention of pre-orders. The window it opened was the one already on the schedule card: everything up to 08:00 UTC on Sunday 6 September, which from the post's timestamp is a little under eleven hours.
What it reversed
The 1 September announcement had a heading of its own, Why not an open beta?, and an answer:
"To validate the stability of our systems properly we will be increasing the number of Steam Playtest invitees in waves rather than opening the beta completely."
So for two days and two hours Beta 02 ran as designed — guaranteed for pre-orders, waves for everyone who had pressed Request Access — and then, for the final night, it did not. Bulkhead called the test a genuine scale test when it announced it, and "come break our servers" is the same idea taken to its end: having watched the waves hold, it removed the gate to see what the servers did with no gate at all.
How big it got
This site's concurrent-player record is read straight from Valve's API on every load of the page. The highest reading it has taken is 244,926 players at once, at 21:04 UTC on 5 September — four minutes before the Steam post's timestamp. The first beta's peak, 105,774, was set on a Saturday too; the second one more than doubled it, and did so on the Playtest application rather than the main store app, so any chart drawn from app 1867240 shows nothing for either weekend.
How the door actually opened in practice — the Request Access button on the Steam page granting the Playtest at once rather than entering a queue — is what players and press report, and it is consistent with the numbers. The post itself does not say.
What it does not mean
It does not mean another test. Nothing is announced between the Sunday close and Early Access at 16:00 UTC on Thursday 10 September, and the servers are down in between, so being unable to connect this week is the expected state rather than an outage. It does not mean a key or a code; none exists. And it was not extended: the beta ended at 08:00 UTC on the Sunday as scheduled, open phase included.
For the record of how access worked across all three tests, the beta page carries it. The release-date page has the launch hour in every time zone, and the live tracker is where to look on the 10th.