These days you can get your reentry permit directly at the airport, and in the case of a PR you can stay outside the country for up to a year. Much better than the “buy an expensive stamp” system they had prior. I moved back to the US at the end of 2018 and haven’t gone back, so I assume my PR expired. That said, if I ever went back I could likely re-obtain it due to my spouse being Japanese.

I’m not aware of anyone trying to game keeping their PR, but I do know that if you need to repeatedly stay out of the country for awhile you need to provide a solid reason (like caring for a sick parent).