Destiny 2 confirmed a number of months ago that cross-play was coming to the game and it appears that this is now live in a beta form. There are a number of reports of people being able to access cross-play across the various platforms. At this moment in time, the Destiny servers appear to be struggling as it is the start of the new season but some people have managed to get in. Dan from Stadiasource and I were able to meet up in Destiny 2. I was using GeForce Now and Dan was using Stadia and as you can see below it worked!
This will be a great feature and should help with the multiplayer lobbies going forward and make matchmaking between players on different platforms easier! Along with Cross Platform progression the cloud no matter what platform will be a great way to play Destiny 2 going forward.