[teruakohatu]

I don't think we have ever seen such a reversal, the closest might be Microsoft's attitude towards open source, but that is apples and oranges.

Facebook used to be more open. You could connect to Facebook Messenger using any Jabber/XMPP. They never reversed course, if anything they made it harder to use Messenger without the application.

[Avamander]

Microsoft would turn back the moment they could, if it made business sense. Right now though, they have to play nicer or get left out.