The following content has been copy pasted from the official Gentoo wiki as per Mircea Popescu's recommendation. This can be expanded into a comprehensive guide for installing Gentoo Linux in such a way as to be compatible with the interests of #bitcoin-assets.
Having trouble with sys-apps/systemd being pulled in your emerges for no apparent reason? Read this article in order to find out how to resolve this problem.
Most packages depending on systemd actually depend on systemd or OpenRC or some other init system. If no other init system is installed Portage will attempt to install the first in the list, which, in most cases, will be systemd.
This is perfectly normal behavior; Portage cannot magically guess which init system the user prefers. In order to help Portage understand that the users have preferences a USE flag can be defined to describe user preferences directly. That is the magic of the USE flag system.
Add ”-systemd” to the system's USE variable in /etc/portage/make.conf.
In order to explicitly inform Portage to never install systemd, a mask file will need to be created. Masks can be created a two different ways. Either create a FOLDER called package.mask in /etc/portage then create individual text files containing lists of packages to be masked OR create a FILE called package.mask and organize it all in one big file. Portage will be happy with either method. The choice is left up to the system administrator.
Create a file named /etc/portage/package.mask/systemd or /etc/portage/package.mask and append the following lines:
Masking udev will not result in a broken system; Portage is smart enough to automatically replace udev by sys-fs/eudev: the systemd-free fork of udev.
Well, it would appear two methods continue to exist for organizational purposes. Some users like having everything defined in one file, while other users like having each set of packages masked in separate folders.
Some packages have systemd as a non-optional dependency, if systemd is masked, you won't be able to install them
Either stop using such packages, switch to systemd or use some systemd emulator/wrapper/whatever
Portage does not activate systemd when installing it, if you do not activate systemd, you will remain on openrc (or whatever init you are currently using)
To remove systemd, follow the steps above and update @world, or remove it manually:
# emerge --ask --unmerge systemd