Support for deploying Office Add-ins or Calendar Apps to Office 365 Calendar
We support Calendar extension points since long time ago, Give it a try!
Larry A commented
The current extensions points for Outlook Add-ins across all platforms are limited to Mail Read,
Mail Compose, Add-in Commands with the exception of Office 2016 for Windows which includes Modules. See https://docs.microsoft.com/en-us/office/dev/add-ins/overview/office-add-in-availability
Would be very helpful to include extension points for calendar and tasks items too.