Re: Er what?
"There's no need to break it [Nextcloud] on purpose, it will get broken anyway just by updating the APIs without telling anybody."
No, because Nextcloud runs their own service. They don't emulate OneDrive, so if Microsoft does change those APIs, Nextcloud will be unaffected. Windows does change, and it's the duty of any company making software to run on Windows to read the updates and deprecations so their software keeps working, but as companies go, Microsoft is pretty good with backward compatibility on such things. Can you point me to a single example where Microsoft broke Nextcloud's integration, intentionally or not?
"Microsoft breaks their own products all the time, so I don't understand the responses here that they wouldn't break a competitor's product, because they wouldn't do anything illegal, because it's not the 1990s anymore, because ... oh shut up."
Break your own product and your users are grumpy. Break someone else's product by accident, and people are still grumpy. Break a competitor's product on purpose, the competition authorities that care (which does include the EU's) can hand out a fine in the hundreds of millions, which they have done before. That's why they try not to break the law.
"What NextCloud wants is an open and stable API as a contract between Windows and client software that doesn't favor Microsoft's own client software."
And does what, precisely? Because most of the Windows APIs do that already. They have to be specific about what they think Windows is doing to negatively affect them. The only thing I can see at the moment is that OneDrive is present on all Windows installations, which they can argue is harming competition, but it's pretty minor. If they did something to prevent Nextcloud from working, the situation would be different, but they haven't and no APIs exist which give OneDrive powers that Nextcloud can't have.