Actually, it’s not totally true. The first exemple I have in mind is Katharine Hepburn and Spencer Tracy. If you’ve never heard about their story, you should just google their names, the similarities with Gillian and David are interesting (minus the tragic ending), and it’s funny that Gillian chose to read one ofr Helpburn’s letter at Letter’s live in 2014. I also have heard a few similar stories of celebrities hiding their relationship for a long time in other countries, I think it was @duchov who told me about a polish couple.
But in the end, every story is different, and there’s no one like David and Gillian, and nothing like their relationship. It’s unique (her words!) and there’s definitely something more than a simple friendship between two costars going on between them, either it used to happen or it’s still happening. (probably both :D)