It's just one big assumption based on one dev's ArtStation post.
Destiny 2 leaked green subclass

Destiny 2 fans are going wild at what they believe to be a leak of an entirely new subclass coming in the near future to their beloved MMOFPS. The leak comes from a very short clip that was posted to a Bungie artist's ArtStation account. The content, since removed from ArtStation but reposted elsewhere, was first reported on by the DestinyTwoLeaks Twitter account.

From a quick look, the footage that runs for less than ten seconds, shows off a Titan performing a scythe finisher on an opponent. The more interesting stuff can be seen on the lower left side of the HUD.

The area that displays the cooldowns on your abilities features green icons. Typically, these icons are different colors based on the current subclass you're using. For instance, they are orange for solar (fire), purple for void, and so on. What isn't in the game, at least not yet, is a subclass that uses the color green.

In addition to those unique green icons, the icon used on the super meter is not used for any existing super in the game. Finally, the melee ability shows that it has three charges available, something else that is also not currently possible in the public build of the game.

The prevailing fan theory is that this was an unintentional leak by the Bungie developer. Fans believe that this leak shows off a poison or decay-based subclass, potentially called "Vapor," that will be added to the game in a new season or the game's next expansion. The idea of a poison-based subclass has been kicking around for a while but has never been announced nor confirmed before.

It's really not unreasonable to believe that the green icons were an indication of a new subclass coming to the game. While Destiny 2 has three "Light" based subclasses (Arc, Solar, and Void), it only has one "Darkness" subclass (Stasis). This leaves the door wide open for potentially two additional Darkness subclasses to be added to the lineup to add some symmetry to the idea of light and dark abilities.

There is also the very real possibility that what was shown off in the clip was just some internal developer testing or options that will never see the light of day. The abilities in the clip, while green in color, did reuse icons from other subclasses. They could be placeholder for an actual new subclass, or they could just be something that the artist was toying around with, or the color green could be something that Bungie uses all the time on developer builds to differentiate it from the live game.

We may find the answer on August 23 as Bungie has a special stream on Twitch and YouTube where they will showcase what's next for the game and franchise.

Apologies for the low-res and compressed image used above. It's not exactly easy getting a high-quality screen grab from a Twitter video.