In June 2019, eBay introduced a new order ID format, but allowed developers/sellers to decide whether to immediately adopt the new format, or to continue working with the old format.
Beginning in June 2020, only the new order ID format will be returned regardless of seller preference.
Unlike in the past, instead of just being known and exposed to the seller, these unique order identifiers will also be known and used/referenced by the buyer and eBay customer support.
Because we realize that some integrations may have logic that is dependent upon the old identifier format, eBay is rolling out this [change] to support a transition period of approximately 9 months before applications must switch to the new format completely.
The new format is a non-parsable string, globally unique across all eBay marketplaces, and consistent for both single line item and multiple line item orders. These order identifiers will be automatically generated after buyer payment, and unlike in the past, instead of just being known and exposed to the seller, these unique order identifiers will also be known and used/referenced by the buyer and eBay customer support.
The above notes are directly from the eBay Developers Program API documentation with minor changes for readability. If you have any questions, please contact eBay seller support.