https://github.com/NixOS/nixpkgs/issues/45071 says the group is needed because systemd udev rules refer to it. eudev installs no such rules.
On a (eudev) desktop system with 1273 packages installed, nothing is owned by the "render" group, and there is no reference to such a group in /lib/udev. Some grep in /var/db/repos/gentoo found references only in systemd and eudev. Maybe systemd default rules use such a user, but eudev clearly does not.
This seems indeed the case. No rules reference the 'render' group.