https://www.gamespot.com/articles/destiny-2-servers-are-down-for-scheduled-maintenan/1100-6453265/

Update 5: Maintenance is finally over, and Destiny 2 is back online. That said, there's sure to be a rush of people hoping to start playing, so Bungie warns that you may again encounter the server queue we've seen at launch and a few other times. As before, don't try to back out of the queue and sign in again; doing so will only cause you to lose your place and require more time before you can get in.

Update 4: While the downtime estimate for Destiny 2's servers has not been changed, Bungie did provide a vague update on what's going on. "Still conducting planned maintenance and addressing some new issues," it said in a tweet. "Please stand by. Pardon our dust." There was no further word on what's causing the delay.