Has human attention span really dropped below a goldfish's?

No. The claim that attention spans fell from 12 seconds in 2000 to 8 seconds, below a goldfish's 9, comes from a 2015 Microsoft Canada marketing report that cited no study for those numbers. BBC's More or Less traced the figures and found no source. Goldfish, for the record, can be trained and remember for months. Attention varies with task and motivation, and there is no evidence of a species-wide decline.

Source: Microsoft Canada, Attention Spans (2015); BBC More or Less (2017)