If you don't own Shadowkeep, Beyond Light, or the Forsaken Pack, you will not be able to access the related strikes when they're Nightfalls, and you will not be able to access or progress their campaigns or quests, including Exotic quests.
The news site is incorrect in the statement because of a misreading of the help article. Bungie has since removed the part of the article that caused the confusion, however the game news site has not updated their information.
You will not keep any expansions you do not own once they leave. This is standard for Gamepass.
You will be able to keep any related equipment you already have on your character or in your vault, but you will not be able to pull any of it from Collections without a valid license.