default or selected kernelversion does not have config value CONFIG_NET_DSA_LEGACY.
Result is shown for kernelversion 6.9.9

Support for older platform device and Device Tree registration

configname: CONFIG_NET_DSA_LEGACY

Linux Kernel Configuration
└─>Networking support
└─>Networking options
└─>Support for older platform device and Device Tree registration
In linux kernel since version 4.2 (release Date: 2015-08-30)  
Say Y if you want to enable support for the older platform device and
deprecated Device Tree binding registration.

This feature is scheduled for removal in 4.17.