A very useful version of it is free. More collaboration features come with paid plans.

https://www.adobe.com/products/xd.html

I see a free starter plan there.

Right, but that's very different from "Adobe XD is free". You wouldn't call AWS "free", would you? (ok, this is an extreme difference, but for all we know, it's only a matter of time until the non-free versions become differentiated enough. It's not like Adobe doesn't know marketing)

The only difference between the free (Starter plan) and paid version is that the free version limits you to sharing one prototype at at time.

Thats it. Otherwise, functionality is the same.