Hello, i was able to ask mailbox.org, which is using Open Xchange and got the following answer:
Currently we do not consider this parameter at all. Instead we are using “calendar-auto-scheduling” as described in https://tools.ietf.org/html/rfc6638#section-3 :
When a calendar object resource is created, modified, or removed from
a calendar collection, the server examines the calendar data and
checks to see whether the data represents a scheduling object
resource. If it does, the server will automatically attempt to
deliver a scheduling message to the appropriate calendar users.
Several types of scheduling operations can occur in this case,
equivalent to iTIP “REQUEST”, “REPLY”, “CANCEL”, and “ADD”
So for the described case OX should send notifications anyway. However the meaning of RSVP=TRUE as we understand it is not to tell the server to send a notification or not - this setting is more to request a participation status reply from the participant (like: “Do you want to participate? yes/no”).
Does this help?