Flat Cube became default only on SQL Server 2016 and newer:
Creating a New Cube
A new cube will be created as a Flat Cube if you mark the checkbox "Flat InfoCube".
The default value of this checkbox has been changed. It is now turned on. You can revert to the old behavior by setting the following RSADMIN parameter (In this case, only the default setting of the checkbox changes):
MSS_DEFAULT_FLAT = FALSE
A Flat Cube always has a columnstore index. You can choose the Flat option in combination with the RealTime option. Keep in mind, that you cannot create aggregates on a Flat Cube.
Transporting a Cube
The flat property of a BW cube can be transported from an SAP source system to an SAP target system once you have applied SAP Note 2550929 - Inconsistent metadata in case of transport of flat cubes non HANA landscape . A Flat Cube in the source system will be created as a Flat Cube in the target system. A non-flat cube will be created as non-flat in the target system. However, the flat property is not changed in the target system when transporting a cube, if the cube already exists in the target system and is not empty (means, the fact tables contain data).
Use SAP report RSDU_REPART_UI for converting a single cube
Use SAP report RSDU_IC_STARFLAT_MASSCONV for converting many or all cubes
Keep in mind, that an automatic conversion to Flat Cube during an R3load based system copy or database migration is not possible. You have to convert the cubes after the database migration on the target system.
Since the conversion to Flat Cube can be very time-consuming, you often do not want to perform this on all your cubes. You may want to start using the Flat Cube for your most important cubes. For all non-flat cubes you should at least apply the Columnstore (which is a quite fast and simple operation). A Flat Cube always uses the Columnstore.