Launch System Preferences
Select Internet Accounts
Select Add Other Account…
Select CalDAV account
You can then configure the calendar to synchronise with. The calendar must support CalDAV to work with macOS.
Indeed, it was a more wide-spread and general issue with Google.
However, I had opened a formal trouble ticket with them, which eventually got recognized, acknowledged, escalated, worked upon and resolved, by about 10:30 am PST 8/23/19.
However, you may need to still purge your calendar cache files:
go to calendar.app, go to settings, go to accounts, ...
You can set a mailbox rule, that when a message is both in trash, and is a Meeting request, move it to the inbox (or a folder/area of your choice).
Screenshot is OSX which I imply that you are using. This would be possible on Windows too with a similar approach.
There is no way to suppress sending notifications from the iPhone or macOS calendar apps. To do this you'll have to go to the Google Calendar web app (https://calendar.google.com), right-click the event, and click Delete.
I can't answer how the event got there, but if you've never declined or deleted it before, that would explain why you're still seeing it.
Assuming this is a Google Calendar, the following advice from https://the-parallax.com/2018/05/11/how-to-stop-calendar-spam/ should help:
Google Calendar: From a Web browser, click on the event in Calendar, but be careful not to reply to it, and not to click on any
of the links within it. At the top, under the Going section, click
“Report spam.” The ...