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What causes this issue?

Corrections with Deleted Methods cannot be shipped via SAP note.
The system has identified that entities in the SEGW project/CDS view
that are used in your Referenced Data Source project have been deleted.
If you choose to regenerate the project, as a result the corresponding
CRUDQ methods in the data provider class would be deleted as well.
SNOTE does however not allow the delivery of changes that contain the
deletion of methods via a correction instruction.
Therefore, the option to stop regeneration has been provided as a
preventive measure.
Example:
The described problem can occur if value helps in a CDS view are
replaced. Technically this results in the creation of new entities for
the new value helps and the deletion of the entities that have been used
so far as value helps.

System Response

You have chosen to stop the regeneration of DPC class.

How to fix this error?

If deletion of entities in your Referenced Data Source project is
required, you can proceed as follows:
1.,,Proceed with regeneration, so that the CRUDQ methods that belong to
the deleted entities will be deleted in the DPC class.
2.,,Recreate the deleted CRUDQ methods manually and add a dummy
implementation for the CRUDQ methods the DPC class. These manual changes
will be captured in SNOTE and can be delivered to customer systems via
correction instructions.
The names of the deleted methods will be shown in a popup and will in
addition be logged in the application log that can be accessed via
transaction SLG1.
You can search for Object '/IWBEP/' & Sub-object 'DESTIM' in Transaction
'SLG1' to get the details.

Procedure for System Administrators

Deletion of entities in CDS views that are used by Referenced Data
Source projects should be avoided in general in a maintenance landscape.

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.