Why Devuan when therer's Antix and MX ?
 

If I understand correctly, the main motivation for starting Devuan was to have a debian based distro without systemd. But doesn't AntiX and MX already do this ? or is it only to a certain extent ? like not having systemd as init, but having systemd anyway for other tasks (systemd-shim).

MX Linux includes systemd-shim, antiX does not.

At the time of the Devuan fork, it looked like Debian was going to be systemd only, but after some 'complaints', they rethought it & allowed for non systemd install, which is what both the above distros use.

Based on my experience with Devuan (I haven't used it in about year), it was not so much a fork of Debian, but a clone. It looked, felt, and acted like Debian, just without SystemD.

I think that is the answer to OP's question.

Devuan may yet be the way to go in the future, if Debian decide to drop all else but systemd.

I've just installed Devuan ASCII 2.1 on a couple of my computers, just to keep an eye on it, & it's running as expected. :)

AntiX is a live CD distro according to their website. Similarly, MX Linux wants to be a mid-weight desktop distro. Devuan wants to be a systemd-free Debian, which I generally think of as a customizable base. I choose Devuan because I want to mix and match the desktop components on my desktops and have a minimal install for my servers.

They may have 'allowed' for this but the use of systemd somehow creates a web of dependencies that in practice block the use of other init.
If you look at the MX19.2 live iso for instance you will find that the version of Gparted that ships with it does not work and the 'error' it reports is that
'PID1 is not systemd'. The same problem leads to partitions on local drives being unreadable by the user. This and other problems lead me to drop MX19.

I do know that, through an expert installation of Devuan, you can choose its init software between SysV and OpenRC and opt NOT to install non-free software.

You can't do that with MX Linux or AntiX.

Like all derived 'distros', they are based on a true distro that compiles the software from source, in their case Debian.

All derivative 'distros' make choices for the end user - so if you want specific software, you usually install from the base distro yourself.

antiX offers a net install.iso (either sysvinit or runit) which does not include non-free software, though the repo is enabled by default.