CONFIG_8xx_COPYBACK is not available for the default architecture x86.
Result is shown for architecture powerpc
default or selected kernelversion does not have config value CONFIG_8xx_COPYBACK.
Result is shown for kernelversion 5.9.8

Copy-Back Data Cache (else Writethrough)

configname: CONFIG_8xx_COPYBACK

Linux Kernel Configuration
└─> MPC8xx CPM Options
└─> Copy-Back Data Cache (else Writethrough)

Saying Y here will cause the cache on an MPC8xx processor to be used
in Copy-Back mode. If you say N here, it is used in Writethrough

If in doubt, say Y here.