What is the evidence that says they are slowing down.

There is a vast outpouring of energy from supermassive black holes and the only mechanisms proposed are that the energy comes from the initial spin. In order to justify this, the researchers then have to posit that more spin is donated by accreting matter over time than is lost.
As I indicated, this is far from conclusive for either them or me.
supermassive black holes it looked like the standard accretion disk model, you will have to explain the differences to me

The crucial difference is the fact that black holes cannot slow down. I then propose that occasionally, neutron stars merge with a combined spin that is unusually low, and these grow to the size they are today. Normal black holes have high spin which limits their ultimate size.
Dave Proffitt