We are in the process of migrating an application that uses the EWS API to the newer Office 365 REST API, mainly because it provides more flexible authorization options. The current application uses extended properties to store information (identifiers internal to our applications) on Event objects.
The Office 365 Message API resource seems to implement extended properties using SingleValueExtendedProperties and MultiValueExtendedProperties fields. Is a similar option available for Event resources?
Extended properties are in preview right now, and they're not currently implemented on Events. We are working on that, so stay tuned. However since this is in preview, bear in mind things could change.