I remember in the older version of Houdini if you hit the “e” key you could toggle visibilty of “everything” / other objects inside the sop level.

Is there a new hotkey or way to hotkey it? :?

It's now bound to ‘y’, though you need to be in the view state in order for it to work.

What do you mean by the “view state”?

view state is Esc in viewport (camera icon on left bar)
or temporary Space key

so space+y is what you are looking for or just y if you are in mentioned view state

or just stay with your mouse over the icon that does this (top right of the viewport) and it will show you the hotkey

Y key it is! Thanks for the help