Does plaster get lighter over time? (After it cures, does it continue to get lighter ... lose water?)?

it does but not forever it is ground rock with a setting agent added Gypsum I plastered for 40 years when wet its the water but a CWT bag as they used to be was pretty heavy , we used to unload lorries by hand you see , I was last one one day did 6 ton on my own its heavy alright just having a laugh it is stable after dry but can be eroded by salts and infestations that turn it back to dust after time if that is what you mean