The mass buff raid on the strats yesterday did not reduce the city to 20% or below but rather left it at 25%.
I'm pretty sure that 20% is the threshold of having an effect on strategic factory regeneration times.
Not according to game FAQ:
Each time a train, convoy or barge spawns, it first checks to see if there has been damage to the supplier (cities supply factories and factories supply fields). The amount of supplies carried to the destination depends on the amount of damage to the supplier. If the supplier has been damaged by 50%, only one out of every six train cars or convoy trucks will carry supplies.