After digging into the code...
Phased Updates is implemented by the file /usr/lib/python3/dist-packages/UpdateManager/Core/UpdateList.py, which is provided by the package python3-update-manager. Look around lines 185 and 360. The actual python code is apt.apt_pkg.config.find_b(), which searches among all apt configs.
- The string should simply say: - Update-Manager::Always-Include-Phased-Updates;or- Update-Manager::Never-Include-Phased-Updates;. A True/False boolean is not needed or checked.
 - You should be able to add that line to any config file in - /etc/apt/apt.conf.d/. Or create your own:- /etc/apt/apt.conf.d/99-Phased-Updates.
 
- The presence of phased updates, or the presence of - Never-Include-Phased-Updatesshould both be logged in your- /var/log/apt/logs when Update Manager (also known as Software Updater) runs.
 
- Other - apt-based package managers (Apt, Synaptic, Ubuntu Software, Unattended Upgrades) don't use Update Manager code, do not use phased updates, and ignore the presence of the config string (it won't be logged).
.
 - aptbegan using Phased Updates starting with 21.04.
 - The - aptsettings are slightly different than the Update Manager settings:
 - Update-Manager::Always-Include-Phased-Updates;
APT::Get::Always-Include-Phased-Updates True;
- Update-Manager::Never-Include-Phased-Updates;
APT::Get::Never-Include-Phased-Updates True;
 
 
- We're talking about spreading updates (usually) across a few days -- for most users that will make no difference. You can get updates early by telling - aptto ignore phasing (One-shot:- sudo apt -o APT::Get::Always-Include-Phased-Updates=true upgrade)
 
For fun: The current status of phased-update packages in the repositories is at https://people.canonical.com/~ubuntu-archive/phased-updates.html
Advice for New Users: The purpose of Phased Updates is to protect you from an occasional buggy update. Let it do that. Advanced users who want to disable this feature know how to recover from those.