You want to migrate "FI – G/L account open/line item" and recognized that this is deprecated.
SAP S/4HANA Cloud 1805.
Reproducing the Issue
Run Migration Cockpit.
Refer to SAP Note 2568909 - SAP Release Note for SAP S/4HANA Cloud data migration content in which you can find the link to the SAP Help Release Comparison of Migration Object Templates where you find the delta information of any migration objects between two releases, this note is also part of the Collective Note and FAQ for the Migration Cockpit: SAP Note 2538700 - Collective SAP Note and FAQ for SAP S/4HANA Migration Cockpit (Cloud), you would also find out some very useful information there about Migration Cockpit.
Regarding G/L balances we have "FI – G/L account balance" and 'FI – G/L account open/line item (deprecated)" in version 1805. In release 1808 you will find the object "FI – G/L account balance and open/line item” that covers both. Below it is marked in red. So there is no loss of functionality.
KBA , CA-GTF-MIG , SAP S/4HANA Data Migration Cockpit Content (LTMC) , How To