Feature-wise, there's very little that distinguished the latest OnePlus 6 from the OnePlus 5/5T. All three phones come with OnePlus' Dash Charge that charges your phone at full speed, even when you're using it while it's charging.
All three phones also have dual-camera systems, and all are capable of taking portrait shots with the rear camera. In fact, you could say the older OnePlus 5 from June 2017 has a feature that the newer OnePlus 5T and OnePlus 6 don't: 1.6x optical zoom. The secondary lenses on the newer OnePlus 5T and OnePlus 6 are only used for the portrait mode effect.
Software-wise, you'll find pretty much everything from the OnePlus 6 on the OnePlus 5 and OnePlus 5T.