Nowadays there is quite some debate and controversy over whether Linux distributions should use
systemd, GNU Shepherd etc. But all these systems are relatively new - for these examples dating back to 2010 and 2003 respectively.
Since the whole point of the term "GNU/Linux" is that GNU was (at that time) a complete operating system without a kernel, what init system was used in early distributions, for example in the 90s? It seems that
init (or equivalent) is a pretty fundamental thing to have in any UNIX-based system. Also, what was wrong with the systems used at that time that led to the creation of
systemd et al?