Facepalm Monday...
My collegue denies to provide breaking changes in our login API in a separate version to the other teams depending on it.

What is the reason for his stubborn rejection?
It's scrum. We haven't planned the effort for realising a versioning concept for our API.
Let's build it in the next sprint as a part of live deployment strategy.

The point he miss is that the ProductOwner wants his API change deployed during the next sprint.
Additionally, it is best practice, having a compatible, deployable product after each sprint, without any risks.
Furthermore, another best practice to provide your API is one URI without a version part holding the current development of the API. And URIs with a version part in it to keep a specific request/response structure and behavior.

What really grind my gears are sayings like 'if the other teams had well programmed their software, modifying our API won't have any effect on them'

C'mon dude. That's far from reality, as anybody knows.

I can't accept, we provide unprofessional API builds, as he is going to do.

So, i have to spend my time and energy to change his mind, together with other software-architects, planning the big thing API-Gateway *sigh*