These minimum SDK requirements have been known for a very long time. It's correct that the email only got sent recently, but the requirements are usually announced 2 years in advance in this manner:

After 0 days: New Android version comes out After 1 year: App Updates need to target the latest Android version After 2 years: Apps can't be downloaded on devices with the "new" Android version anymore unless they target the "new" Android version

So normally there should be plebty of time to prepare. However, if you're an indie developer or not actively maintaining the app, then it definitely is annoying, since usually the apps would work perfectly fine on the new Android version without updating the targetSdk version.