[neuromanser]

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...

[ufo]

Is that only for command substitution?

[neuromanser]

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.