The plugin is not showing up in the “installed plugins” window for 4.20, and as I said doing “install to engine” In the vault has no effect. The same is true when I try upgrading a 4.19 project and get the “this project requires plugin x, would like to download it from the marketplace?” dialogue. I’ll try reinstalling 4.20 and see if that fixes it.
Update: reinstalling 4.20 didn’t help.