Yesterday, the U.S. Supreme Court handed down its decision on Google v. Oracle. That case dealt directly with the copyrightability of API's, and whether the owner of the copyrighted work (software program) could preclude another from using the same API call statements in a different application in order to promote interoperability.
TL/DR verison: The Court held that API's are copyrightable, but it is "fair use" for others to use the same API call statements in other programs without obtaining permission from (or paying a royalty to) the copyright owner.
Longer version: In the U.S., a software developer does not need to ask permission from, or pay a royalty to, the owner of another program (e.g., the owner of a closed-source application) when the developer copies the API call statements in the other program and incorporates those API statements into their own program. Note, you can incorporate the names, syntax and parameter lists that is the interface, but you still have to write your own underlying code that lies behind the API. In its opinion, the Supreme Court distinguished API's from regular copyrightable content precisely because of the functionality of API's, and because interoperability promotes the creation of additional works (which is at the heart of the copyright law).
This decision affects the U.S. only. Even so, the case will have a significant impact within the U.S. software development community, and open source in particular.
You can read the Court's opinion here.