And I mean true slow flight, that is flying with the stall warn horn first sounding intermittently and then even slower with the horn sounding continuously.
Surely this is the very thing we are teaching students to avoid! Flying aircraft slowly is potentially dangerous because it can lead to the stall and the spin and is taught as one of the symptoms of the approaching stall; thre point at which the pilot should initiate recovery action. Teaching people to fly around at these speeds means they are less likely to recover from a worsening situation because they "know it is OK" they have done it regularly! It is much better to be comfortable with stalling at a safe height; be familiar with the recovery action, then stay away from the conditions that might put you there in a lesss desirebale position i.e. a height where recovery may be compromised.
Can (should?) a PPL practice entering and recovering from spins?
Provided you know what you are doing, have an appropriately certified aircraft and have had it demonstrated and practiced before, then there is nothing to prohibit it. The choice of a prachute is always debateable but remains optional.