The primary developer of Cin was Thaddeus Kowalski, who worked at Bell Labs but was not part of the UNIX development team. When the manual for UNIX v10 was published by Saunders College Publishing, the man page and tutorial for Cin were included, indicating that they planned to include Cin in UNIX v10. However, the archives of v10 that I know about do not contain Cin, only the man page and tutorial. So it seems that the Cin project is lost.

Cin man page TXT 16 KB