Objective-C++ was introduced in the early aughts (https://luminaryapps.com/blog/objective-c-a-history/index.ht... says 2002; I remember watching the WWDC video in which it was introduced but I'm having trouble finding it now, sorry).
And just not to be a random dude in the Internet, here are the NeXT documents straight out of 1993, page 173.
http://www.bitsavers.org/pdf/next/Release_3_Nov93/Object-Ori...
Page 31 on "Developing business applications with OpenStep"
https://archive.org/details/developingbusine00gerv/page/n7/m...
Finally, from the language authors themselves, chapter 15, Objective-C++ (1989-1990)
"The Origins of Objective-C at PPI/Stepstone and Its Evolution at NeXT"