I personally think there is nothing damaging with 60.
Reference this.
biochemistry-glucose-and-your-brain
40 seems to be the bad number.
I think the “below 70” idea is overly-cautious advice, perhaps based on the idea that, “If we say ‘below 70’, that might prevent people from being below 50…”
The only risk might be if someone spends a lot of time at 60, they would not “feel” it as much, and their built-in personal feelings for a “low warning” would be diminished. I don’t know if that is true, but I have heard that. I still feel lows when I am actually low.