A direct port of the Game State Management sample for XNA 4 that used to be found here (but is now found here), to Monogame. It seems to have since been moved or deleted, but the license can now be found at the new link.
There is a very good XNA Game Studio Archive repo that has a lot of XNA Game Studio education content, including the original version of this repo, if you need more than this.
This is currently working while targeting netcoreapp3.1 and the version of Monogame that comes with the template found here - MonoGame (.NET Core)