Update: I believe the issue is that this feature requires the organization be at 9.2.22064.00044 and my organization (which is in North America) is at 9.2.22063.00196. I've posted a comment on GitHub asking to add this to the preview documentation.
I'm following the Docs for the preview feature of Dependent Assemblies: https://docs.microsoft.com/en-us/power-apps/developer/data-platform/dependent-assembly-plugins#use-power-platform-tools-for-visual-studio
In PluginRegistrationTool.exe there should be a drop-down option from Register called Register New Package. That option is not showing for me.

I've pulled the latest from NuGet. I've confirmed that the version I have running matches the version in the NuGet package on NuGet explorer: https://nuget.info/packages/Microsoft.CrmSdk.XrmTooling.PluginRegistrationTool/9.1.0.153
Explorer says the AssemblyFileVersion is 9.0.0.9658. That matches what I see in the about screen in the tool.

I deleted and reinstalled the tool from NuGet, just to be sure I had the latest.