Copying my comment from SMT issue here:

It would be great to have this feature eventually added to Fossify Calendar, but what I consider a necessity is to preserve this status when it is set by other apps that support it.

An example of what I mean:

1. I create an event with status "Tentative" in Thunderbird.
2. The event gets synced to my Nextcloud server - there I can see the status as tentative using Calendar app.
3. The event gets synced to my phone - in Fossify Calendar I cannot see the status, but I can edit the event.
4. I edit the event time on my phone and save it.
5. The event gets synced to Nextcloud (and Thunderbird) - the tentative status is gone, instead both apps show the status as confirmed.

If I understood your request correctly, most of those features should already be supported. I did a few tests with a calendar synced via DAVx5. As those were just some tests I cannot say how reliably this works in daily use.

Features that worked in my tests
If an event has at least one attendee in addition to yourself the app shows a small circle on the bottom-right side of the profile bubble for each attendee:

• green with a checkmark (confirmed)
• yellow with a questionmark (tentative)
• red with an x (cancelled)

It seems to take at least one synchronization with your calendar before the status icons appear. Additionally, if one attendee has not replied to the invitation yet, the app will show no status (circle) for that attendee.

You can change your own status by tapping on "my status" in the list of attendees. When you set it to cancelled the event's title is striked through in most views (I didn't check all of them). The changed status was also correctly updated on the server.

Thank you, @min7-i , for looking into this and pointing out the attendance feature.

I believe this is similar to (but not identical with) the requested feature and can be used as a work-around in many use cases.
(BTW: I believe it is sufficient to add yourself to the event as an attendee and save and close the event; opening it again, the "my status" (not sure about the translation) field appears - so no need for a different / dummy attendee...)

Personally, I wouldn't be completely satisfied with this just yet, especially because of the different handling on other platforms / in other apps:
This attendance status makes use of the PARTSTAT parameter of the ATTENDEE property (see RFC 5545 #3.8.4.1), whereas the request was to use the STATUS property (see RFC 5545 #3.8.1.11).
This leads to events "cancelled" via Thunderbird / Nextcloud (and shown with strike-through there) not being marked (shown without strike-through) in Fossify Calendar - and vice versa, see screenshots:

• Fossify Calendar:

• Thunderbird:
  

• Nextcloud:
  

Furthermore, when cancelling an event via Thunderbird / Nextcloud, i.e. via the STATUS property, an event update will be sent out to all participants (and shown in their calendars accordingly, if supported). Setting the PARTSTAT parameter of the ATTENDEE property will only be reflected in the user's own calendar, however.

And finally, as @chesio, has pointed out, Fossify Calendar (re-)sets the STATUS property to STATUS:CONFIRMED whenever an event is edited (totally unrelated to STATUS or ATTENDEE), thus overriding anything set via Thunderbird / Nextcloud etc.

For those reason, I would very much appreciate Fossify Calendar (also) supporting the STATUS property.
Again: thank you!

Thanks a lot for taking the time to elaborate on this, @akki42. I see the differences now.

I implemented this feature in this PR: #200