It's still working. I got 4 businesses with unlimited supplies now.
CEO is optional by the way. Unregistering the MC is enough.
1. Shut down the business
2. Unregister MC and exit the business
3. Enter the business (you should see no staff)
4. Go to notebook, Resupply
5. Unregister MC, exit the business. Wait outside the business for 10 minutes.
You can do whatever you want, but stay in a freeroam (don't joint Heists/Races etc). VIP Work / Import/Export, Gunrunning - all these activities are fine.
6. You will receive the supplies arrival notification. Navigate to your business
7. Repeat the cycle since step 4 (YES!!! Resupply again) unless you see a message that no supplies are required at this point.
Don't trust your eyes! Don't look at the supplies bar. The message is the only 100% indicator that supplies are full.
8. "No supplies are required at this point" => Restart the business and disband the MC as soon as possible.
9. Exit the business, wait 15 min in freeroam. You can be a CEO if you want
10. Very important step IMO: Enter your appartment (NOT the business) and register as the MC president. You should see a message that the staff arrived. Unregister MC and exit the appartment.
11. (Optional) Wait another 10 min and navigate to your business. At this point the supplies should be stuck at max.
It may require several attempts.