From the toxicity section (https://en.wikipedia.org/wiki/Borax#Toxicity) it seems like the concern is most about ingesting borax in large quantities or breathing in the dust. And also being cautious on potential health affects:
Since people are already exposed to boric acid naturally through their diets and water, Health Canada advised that exposure from other sources should be reduced as much as possible, especially for children and pregnant women. The concern is not with any one product, but rather multiple exposures from a variety of sources.
"Although it does not appear to be carcinogenic, studies in dogs have reported testicular atrophy after exposure to 32 mg/kg bw/day for 90 days. This level is far lower than the LD50."
The base toxic dosage (on the order of multiple grams per kilogram bodyweight) is the reason it's typically labeled as very safe.
The labels and general knowledge passed from trainer to trainee insist on the reprotoxicity of the flux, which as you point appears to be at exposure levels far below toxic levels. I wonder if the safe label of boric acid for feet is made looking only at the general toxicity of the product, leaving aside potential reproductive issues that might arise with repeated use of small quantities?
