This era has been underway for at least 20 years. AI potentially exacerbates the problem by dramatically reducing the cost of bullshit production, but it also holds the potential to act as a positive agent that reviews and validates available information.

There are probably many significant milestones on the road to 'truth decay', but satirist Stephen Colbert crystalized it in 2005 with the concept of 'Truthiness' https://en.wikipedia.org/wiki/Truthiness