app-emulation/virt-manager[gtk] depends on net-misc/x11-ssh-askpass.
First, this is not a hard dependency. Some people use virt-manager to manage localhost, and thus don't need SSH access. Some people use SSH, but use public key auth, and thus don't need SSH_ASKPASS program either.
Second, net-misc/x11-ssh-askpass is just one SSH_ASKPASS program implementation. There're several others in Gentoo available, like lxqt-openssh-askpass, ssh-askpass-fullscreen, etc.
Third, net-misc/x11-ssh-askpass is an ancient program that pulls in imake build tools. The imake build system includes the infamous cleanlinks program, that can wreak some havoc if run accidentally. I've been bitten by this myself.
A good solution IMO would be to suggest some SSH_ASKPASS programs in the "after installation" message, like I've seen with some other ebuilds.