It only applies to G Suite. If you have a home account, which is almost certainly the case, this won’t apply to you. It may eventually but they have made no announcement along those lines for home users. Only G Suite (i.e. companies and organizations). If you don’t pay for Google and you don’t get an account through your employer you have a home account.
I was having the same question. Although I am not impacted directly, I am figuring the developers can come up with a way to do oauth possibly also.
I received the announcement because I do use gsuite but not with my openHAB install.
Absolutely. The Google Assistant integration uses OAuth. Many other bindings do as well.
Remember, if it came down to it you could always self-host your own CALDAV server with something like Radicale.
Google (and other proprietary and commercial interests) have proven time and again that they cannot be relied upon long term. And then there are the privacy/spying issues…
Therefore I personally decided to start self-hosting and using as much as possible open protocols and (sometimes paid) services. But then again I personally value privacy much more than “convenience.”
So, you can wait until they shut it down, or you can start taking steps now to reduce your dependence… Just a thought… 
Indeed. If you want to replace more than just Calendar among Google’s services, Nextcloud or OwnCloud are two decent options which can replace Drive, calendar, and more.