According to https://github.com/telefonicaid/fiware-orion/blob/develop/doc/manuals/deprecated.md, XML has been removed in Orion 1.0. Does this mean Orion 1 can not be used to send notifications to IBM Proton CEP?
Proton CEP supports JSON but not NGSI format. Is this correct?
That's correct: XML support was removed in Orion 1.0, after being deprecated in Oron 0.23.0 (i.e. support was removed more than half a year after being declared as deprecated).
Thus, you have the following alternatives:
Wait until Proton CEP implements JSON support (it would be great if Proton development team could provide a date on that).
Use Orion 0.28.0 (the last Orion version that supports XML). Have a look to this piece of documentation about using old versions.
Implement a JSON-to-XML software to convert from one format to the other.