The systemd program is a wrapper around the Linux kernel that intends to cover the functionality of init, and much more. Since Debian 8 Jessie adopted systemd as the default init system, the Devuan community was born to keep freedom of choice of init systems.
The systemd controversy illustrates a paradox in the free software community where a single software breaking the UNIX philosophy of doing one thing, and do it well, ends up conflicting with other free software and, in the case of systemd, imposing itself to the developers, and soon taking over the dependency tree of a lot of software beyond its own scope.
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The without-systemd wiki gathers practical knowledge regarding this controversy, notably how to replace systemd functionality.