> Nothing is preventing anybody to re-implement the external API's of systemd on any OS (well need D-Bus).
D-Bus does not exist for any other kernel. Attempting to replicate the API's of such a constantly growing, expanding, and feature increasing API that replaces stable internal daemon, network, mounting, and security features would be far more destabilizing that systemd's own growth.
The result is that cross-platform support now requires publication of several sets of init tools, typically SysV scripts for other platforms and systemd for modern Linux distributions. I'm afraid it's discouraging cross-platform work, which is unfortunate.
>> The default was changed to enabled in the systemd source code in release 230. Unless distributions or developers manually patch their configurations, this unrequested behavior is now on by default.
> There is a configure flag. Anyway, systemd needs to be integrated inside the distributions, it's definitely not advised for random users to compile it themselves without using the distro patches.
The default for the configure flag was changed. Previously working builds would now actively break common user tools, such as screen, tmux, and nohup, without leaving any systemd log of when they were killing such processes.
Like many other systemd architectural changes, this broke numerous long stable tools for no particular benefit. The normal way to kill long-running processes belonging to absent users is a cron job that emails the adminisator. Such tasks normally also allow exceptions for adminstrative users, and have even been written with alternative policies for unattended use or with warning notes to the particular users before killing their tasks, along with a log of the worst offenders.