Been there...
I thought my API keys were expired, I regenerated them, changed a couple of things, checked all API calls to see if they changed API itself...then I searched the exact error and found out.
For such a breaking change to the API, was it hard to drop an email to every account not meeting the damn "requirements" with an API call performed in the last x months, to alert of the change?
Yes, pretty much that. Plus some configuration might be easier with a DNS hosting. But the main benefit is decoupling domain and DNS for easier change.