I want to know the answer to the following question:
What systems used IBM866 encoding?
All the background I'm provided with is that "Nevertheless, the spread of MS-DOS and OS/2 (IBM866), traditional Macintosh (ISO/IEC 8859-5) and Microsoft Windows (CP1251) created chaos and ended by establishing different encodings as de facto standards, with Windows-1251 becoming a de facto standard in Russian Internet and e-mail communication during the period of roughly 1995–2005.". Is that enough to answer the question?
Answer: yes