Maybe it just semantics, but slaving an amp has historically been using the line out of an amp, off of the OT secondary speaker taps, getting a line level “post power section” signal, capturing a line level signal of both preamp and power amp. This would allow the W/D/W rigs or allow big bands to literally power the big wall of cabs in a huge venue controlled by the controls of single amplifier. A solid state amplifier can be used for the W/D/W or auxiliary cabinets because you are accurately replicating the sound of a tube power section.
At some point in the modern era, people like to call using the loop to go into another power section as slaving, but it just an accepted terminology due to lack of a replacement term rather than being true slaving.