Python ecosystem
If you feel like running arbitrary code downloaded over the internet (pypi doesn't support gpg signatures anymore), mat2 is available on pypi, and can be installed like this:
pip3 install mat2
GNU/Linux
Optional dependencies
When bubblewrap is installed, mat2 uses it to sandbox any external processes it invokes.
Arch Linux
Thanks to Francois_B, there is an package available on Arch linux's AUR.
Debian
There is a package available in Debian.
Fedora
Thanks to atenart, there is a package available on Fedora's copr.
We use copr (cool other packages repo) as the Mat2 Nautilus plugin depends on python3-nautilus, which isn't available yet in Fedora (but is distributed through this copr).
First you need to enable Mat2's copr:
dnf -y copr enable atenart/mat2
Then you can install both the Mat2 command and Nautilus extension:
dnf -y install mat2 mat2-nautilus
Gentoo
mat2 is available in the torbrowser overlay.