As IBIA reports, a standard IMO format for reporting non-availability of compliant fuel oil has been developed, as well as guidance for how such reports should be investigated by authorities. However, there is no resolution yet about how to deal with any bunkers that are over the 0.50% sulphur limit, which remain on board a ship when the carriage ban takes effect.
Many member states emphasize that it must be debunkered at the first port of call, but IBIA claims that in reality this may not...
https://safety4sea.com/ibia-obscurity-remains-regarding-2020-non-availability-situations/