Output from apt:
The following packages were automatically installed and are no longer required:
What package install this? Is it OK to remove or to keep it. And what I can use it for?
It might help to know the actual apt command used that generated this output.
On my (mostly) fresh install of Ara, all that exists of greenbone-security-assistant is a residual-config. apt-cache shows no reverse dependencies for greenbone-security-assistant or greenbone-security-assistant-common. Perhaps the installer installed the package and then uninstalled?
Without knowing the precise history of installs/uninstalls/removes/purges/etc you’ve done it’s tough to say but as a rule, I’ve found that if apt reports they are no longer required it’s generally safe to let autoremove do it’s job.