Bash allows the $(< file) syntax instead of $(cat file), and I think the latter might be converted to the former.

It's built-in in Zsh: The substitution ‘$(cat foo)’ may be replaced by the faster ‘$(<foo)’.

https://zsh.sourceforge.io/Doc/Release/Expansion.html#Comman...

Is that only for command substitution?

What do you mean? It's specifically for $(cat …) / $(< …): the latter is a faster equivalent of the former. Other than that, see my comment about $READNULLCMD.