If modifications are done on Out of Box Schemas, what is engineering stand on such customizations, and will R&D support in future issues related to these modified OOTB Schemas/workflows?
The OOB Schemas and Workflows must not be modified. (Schema can be extended using schema extension mechanism).
That rule must be respected by all the customers.
The customizations need to be modified to respect that rule before being deployed in production.
That kind of modifications are dangerous and the customer will have functional and upgrade troubles (conflict and deletion of a part of the customization).
Engineering is not supporting any issues coming from OOB Schemas and Workflow modifications.