Definition - What does Open System mean?
Before the popularity of the Windows OS and the PC, open systems used to refer to computer systems with Unix-like operating systems that accepted any modules or programs from any third party that used the same standard, as opposed to that era's closed systems such as IBM computers.
Techopedia explains Open System
However, the popularity of Windows and the PC means that third-party software and hardware is often interoperable with these systems, and most software made for Windows is portable to any other computer system running other versions of Windows. Because of this interoperability and portability, and despite Windows being a proprietary OS, a computer running the Windows OS can be considered as an open system.