The comment "Operation of the Ethernet MAC and switch is not supported for OPP50" on page 118 of the data sheet, is a restriction imposed when VDD_CORE OPP is configured for OPP50. See Table 3-6.
This restriction is a result of the CPSW subsystem requiring VDD_CORE OPP to be configured for OPP100 and is not a function of the VDD_MPU OPP or the processor operating frequency.
Regards,
Paul