MINIX originally was developed in 1987 by Andrew S. Tanenbaum as a teaching tool for his textbook Operating Systems Design and Implementation. Today, it is a text-oriented operating system with a kernel of less than 6,000 lines of code. MINIX’s largest claim to fame is as an example of a microkernel, in which each device driver runs as an isolated user-mode process—a structure that not only increases security but also reliability, because it means a bug in a driver cannot bring down the entire system.
In its heyday during the early 1990s, MINIX was popular among hobbyists and developers because of its inexpensive proprietary license. However, by the time it was licensed under a BSD-style license in 2000, MINIX had been overshadowed by other free-licensed operating systems.
Today, MINIX is best known as a footnote in GNU/Linux history. It inspired Linus Torvalds to develop Linux, and some of his early work was written on MINIX. Probably too, Torvalds’ early decision to support the MINIX filesystem is responsible for the Linux kernel’s support of almost every filesystem imaginable.
Later, Torvalds and Tanenbaum had a frank e-mail debate about the relative merits of macrokernels (sic) and microkernels. This early history resurfaced in 2004 when Kenneth Brown of the Alexis de Tocqueville Institution prepared a book alleging that Torvalds borrowed code from MINIX—a charge that Tanenbaum, among others, so comprehensively debunked, and the book was never actually published (see Resources).
https://www.linuxjournal.com/article/10754
See also: https://en.wikipedia.org/wiki/Tanenbaum–Torvalds_debate
It’s still used tho.