Four commits were recently posted to the Android Gerrit source code management that are titled "Carrier restriction enhancements for Android Q." The feature will allow for lists of "allowed" and "excluded" carriers, enabling a list of carriers that will and won't work for a device. It even allows for blocking of MVNOs running on your main carrier's network.
Also notable is that with Android Q, carriers will be able to lock the second SIM slot of a dual SIM device unless there's an approved SIM card in the first slot. This restriction is applied immediately and will persist ever after a phone restart or factory reset. Currently, carriers can set individual restrictions for each SIM slot of a dual SIM device.
It's never exciting to hear about new restrictions when it comes to the ways you use your phone, so these Android Q carrier restrictions are kind of a bummer. While Android Q isn't yet official and probably won't launch to the public until later this year, you may want to consider an unlocked phone as your next daily driver if these upcoming Android Q carrier restrictions bother you.