I’ve read the patch notes for beta 10 but still can’t find the Package Manager in the editor, there is no shortcut in the “Windows” tab.
I had the same thing happen when opening an existing 2018.3 project in 2019.1. I restarted the editor and then Package Manager appeared in the Window dropdown where you’d expect.
Certain compilation errors in your project can prevent the package manager UI from loading. We have a change to the way the package manager UI is loaded to address this behaviour coming to 2019.2.