The rule has always been if you are on OPT you need to work for min 20 hours per week. But it seems like this is no longer the case?
https://www.uscis.gov/working-in-the-united-states/students-and-exchange-visitors/optional-practical-training-opt-for-f-1-students says: If you are authorized for post-completion OPT, you may work part time (20 hours or less per week) or full time.
https://www.ice.gov/doclib/coronavirus/covid19faq.pdf (Page 14, Question 8) also says: For the duration of the COVID-19 emergency, SEVP considers students who are working in their OPT opportunities fewer than 20 hours a week as engaged in OPT.
Follow up question, since it says, 'for the duration of the covid-19 emergency', any idea how long will we have once the govt decides the pandemic is over to get back to 20 hours per week workload again without falling out of opt requirement.