Nautilus-actions configuration tool, which was needed in order to have context menu actions in Nautilus doesn't seem available in the Software center in 18.04 LTS.
(This seemed reported as a bug here - but confusingly specifying
gedit instead of the correct package.)
This is not a Gnome-Nautilus feature, I think, as suggested in a comment, because I have seen the same Nautilus version 3.26.3 in Solus Linux, and there Nautilus Actions is available and usable. Solus provides also in its repo a different program called FileManager Configuration Tool, run with
fma-config-tool, which excepting the name is identical and can be used only alternatively to the Nautilus actions usual tool. But none of them is available in Ubuntu, no matter the repos enabled in software sources settings.
How can Nautilus Actions tool (or its alternative) be installed in Ubuntu 18.04 in order to get the custom context menu actions?
Please do not answer by proposing other file managers.
I know that there are file managers that have custom actions - in fact all those used by the main Linux systems and by the main systems of the Ubuntu family have them: Caja, Nemo, PCManFM, Thunar, Dolphin, Pantheon-Files. But I am not asking about that here.