I've yet to find a single scientist saying they had seen an FCS on a SARS related CoV before SARS-CoV-2. Can you find one?
When I initially saw the viral genome, in January 2020, I noticed a pretty esoteric thing. The virus we would later call SARS-CoV-2 has something called a furin cleavage site (FCS).
The FCS, which had never before been seen in a SARS-CoV-2-related coronavirus, is a genetic feature that makes it easier for a virus to infect human cells. So, when I saw this FCS on SARS-CoV-2, I knew it was going to make it easier to infect cells and easier to evade the immune system by tunneling into neighboring cells.
Knowing FCSs were not found in nature in these SARS-related viruses I also knew that for years scientists have been artificially putting new FCSs into viruses to see what they do, and in every case I have seen, in which they published their results, it has increased infectivity, transmissibility, pathogenicity, or a combination of them.