You mentioned Hepburn and Tracy. They were in a relationship for 26 years (and living together for the last 6 or 7) and while everybody in the movie industry and the press knew about it, the public had no idea. This was possible because the production companies in those days had total control over PR and since Tracy as a devout Catholic couldn’t/ didn’t want to get a divorce, the affair was kept a secret. Mind you, in those times there were no social media so it was much easier to keep secrets.