Looks like !$ is a readline feature and $_ is a shell variable?

Yes. $_ is documented here: https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash....

And !$ is documented here: https://www.gnu.org/savannah-checkouts/gnu/bash/manual/bash....

I don't think !$ is properly understood as a readline feature. It's a sh/bash abbreviation.

!! -> last whole command

!!:1 -> first argument of last command

!!:$ -> last argument of last command

!$ -> abbreviation for above