Why are some policy settings missing from Group Policy Management Editor?

Check the Administrative Templates first. The editor displays settings based on the ADMX definitions available to it, along with the corresponding language files. Missing or older templates can explain why a documented setting doesn't show up.

Also check the requirements for each setting in the documentation. A setting may be intended for a different Windows version, so seeing it in the editor doesn't necessarily mean it will apply to every computer in your test OU.

Look for a central store at the domain's SYSVOL share, under the domain folder, then Policies and PolicyDefinitions. If that folder exists, Group Policy tools normally use those definitions rather than the local ones in Windows\PolicyDefinitions. If there isn't a central store, check the local folder on the machine where you're running the editor.

Compare the templates there with the appropriate Microsoft Administrative Templates package. Make sure the matching language files are present too. Be careful updating a central store because it affects what other administrators see when they edit policies.

One terminology wrinkle: the individual entries under Administrative Templates are policy settings, not separate GPOs. You can configure several settings inside one GPO, link it to the test OU, and check the results there before expanding the rollout.

Not every template comes with Windows. Some applications, like Chrome, provide their own ADMX files for managing their settings. That probably isn't the issue for your User Accounts example, but it's worth knowing if other application settings are missing too.