The Destiny 2 loot cave isn't as useful as the original game's legendary grinding spot, but it is a fun little Easter egg (which is itself a nod to another Easter egg). The new loot cave can be found in the same spot in the Cosmodrome, which was added to Destiny 2 with the release of the Destiny content vault, and it's worth making the trip even if you don't plan on farming purple engrams.
The Destiny 2 loot cave can be found in the southern region of the Cosmodrome on the west side of The Divide. From the Steppes landing zone, turn around and head through the factory. Follow it until you reach an open space, then head to the first building on the right side. It looks like this:
Once you're inside, look for the shadowy spawning point in the back annex. Tada! You've reached the Destiny 2 loot cave. Enemies will spawn in this area infinitely, and you can kill them for unlimited rare, legendary, and even Prime engrams. However, compared to the original loot cave, this is a much less effective way to gather loot. The first loot cave was born out of desperation, but the Destiny 2 loot cave is more of an AFK stop-gap. Between updated Power scaling, the scarce but unlimited powerful drops now available through playlist activities like Strikes, and Ghost mods that make those playlist activities even more rewarding, this isn't a great way to get loot. But hey, if you want to play the game by not playing the game, we won't stop you. That's what the loot cave is all about, after all.
Speaking of the original loot cave: you can find a Destiny 2 loot cave Easter egg over in the Skywatch section of the Cosmodrome. It's the same one that was added to Destiny 1 in its later years. Just walk straight ahead from the Skywatch landing zone and look for a cave by the nearest Hive spire.
Inside you'll find a pile of bodies with an interactive prompt which will play the line, "A million deaths are not enough for Master Rahool," a grim reminder of the farming seen in the original loot cave.