As far as I understand it, it is a general way for meson to prevent the addition of features just because the required source has been found. Each feature must then be explicitly enabled.

I checked if the setting is effective by creating sysupdate. If auto-features=enabled, it was enough to add util-linux to the PACKAGECONFIG[sysuser] depends field and fdisk support was built without having to explicitly set -Dfdisk=enabled. With '--auto-features=disabled' the latter was required.