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.