How do I get text key binding "yankAndSelect" to work?
Update: It seems that on some versions of macOS, setting the configuration globally doesn't work; instead, use defaults write com.apple.TextEdit NSTextKillRingSize -string 6 (replacing com.apple.TextEdit with the desired application and 6 with the desired kill ring size) to set it on a per-app basis. Additionally, it appears that NSTextKillRingSize must be no greater than 16 or it will not work.

Original answer:
The issue appears to lie in how the defaults CLI handles the global defaults domain. Specifically, you want to write to the correct host's global defaults (i.e., yours), which it seems doesn't occur by default. (I still don't fully understand the intricacies of how all this works.) To accomplish this, replace your current defaults write command with the following:
defaults -currentHost write -g NSTextKillRingSize -string 6

This worked for me on Big Sur 11.1.