Just copy the legend in the layout.. and whoa the link is broken, no more updating legend.
You can always re link it to the viewport again though.
After you copy you see a little warning here to let you know it is broken
just click the button to re link it. Be careful though... you ahve to make all your changes again!
If you want to use the same legend on a few layouts in that drawing, turn it into a block and insert it across all the layouts.