I tried to disable the billing for the app engine but this prevents to use it because the free storage quota is 1GB and we have 6.2GB. So I restored the billing for now.
Instead I would urge everyone who has still pending reviews (and not yet converted into merge request) to take action as soon as possible.
The plan is now to make the data store readonly by the end of the month, make a copy of the data and then disable the billing. This way in case of need we could still re-enable the billing exceptionally to retrieve some missing information.