I don't know how it was diagnosed in the 1980s but the test Trump brags about acing wasn't developed until 1996.
I imagine it was easier to hide Reagan's decline because of the lack of social media. Even Biden's decline was hidden better than Trump's because he didn't use it constantly. But given Trump's rise to power was (and continues to be) greatly aided by a neverending stream of train of thought tweets having them stop or change tone now would just trigger speculation that he is incompassitated in some way That's already happened a couple times when he has gone quiet for a couple days.