For me the cause of this code was an aftermarket
converter (TCI Streetfighter) which just has too
weak/small a clutch, to hold especially when the
commanded line% is zero (as is the case for the
stock F-body tune, even up to 20% or so engine
load, by my logs). Force motor changes at low line
got me to "tolerable" slip but still needed to
raise the slip RPM threshold to keep from getting
hit on long light grades.

I don't think GM considers aftermarket components'
shortcomings, in their diagnostic trees. But the
smaller torque converters are in my opinion the
most likely of causes, especially small cheap ones
with clutches off a 4-cylinder model.