Is Thunderbolt 2 forward-compatible to Thunderbolt 3?

I just found something that sheds some light on my question. On the Apple Thunderbolt 3 to Thunderbolt 2 adapter page they mentioned that the adapter is bidirectional and I quote:

The Thunderbolt 3 (USB-C) to Thunderbolt 2 Adapter lets you connect Thunderbolt and Thunderbolt 2 devices — external hard drives and Thunderbolt displays, for example — to any of the Thunderbolt 3 (USB-C) ports on your MacBook Pro. As a bidirectional adapter, it can also connect new Thunderbolt 3 devices to a Mac with a Thunderbolt or Thunderbolt 2 port.*

This means that the adapter may be used to connect the new MacBook Pro to TB1/2 devices or an older Mac to a TB3 device. This definitely gives me the answer that I seek about forward-compatibility. However, this may come with some caveats.

• For one, the page states that forward-compatibility requires macOS Sierra so it may require specific drivers that comes only with the update.
• Compatibility of this adapter with Windows is still unknown at this point (though I never mentioned that in the question anyway)
• Speed will be affected by the slowest connection... though that's expected.
• Other adapters may not support bidirectional connections so it's best to read the spec sheet or ask the manufacturer, if possible.

I will try to get my paws on this adapter and a Thunderbolt 3 device when they arrive in this part of the woods and try them out. I'll then update my answer depending on what I find accordingly.

UPDATE: I found a video on YouTube on how to connect a Thunderbolt 3 capable NAS to a Mac Mini with Thunderbolt 2 and a follow-up video tests the speed of the makeshift connection. This proves that Thunderbolt is forward-compatible, indeed.

There are a few things to note though:

• The NAS is powered externally. We don't know if it will work with devices that rely on power from the cable itself.
• No word on Thunderbolt 1 to 3 forward-compatibility but it should work, theoretically.
• He didn't seem to mention the version of the OS in the Mac Mini. Also, still no idea regarding Windows compatibility.
• Will USB-C (non-Thunderbolt 3) devices work? I think it will, but we'll need actual tests to prove it.

I will update this answer further once I get my hands on devices that I can test myself.