With only 10% of the population, we might find so much institutional knowledge lost that the remaining just can't feed themselves. We'd have to relearn hunting and gathering, and that just might not happen.
If we have to relearn hunting and gathering, that's not even closely compatible with feeding 10% of current human population, that's consistent with <1%, probably 0.1% or less of current population surviving. Feeding even 0.8 billion people still requires intensive mass agriculture.