Checkbox does what you would expect. Press tab from the amount field to get there.
It’s unchecked by default.
When launched with -addresstype=bech32
it’s checked by default. When launched with -addresstype=legacy
it unchecked and disabled.
The change in receivecoinsdialog.ui
is smaller than it looks, due to the way git handles XML diffs. I had to add a horizontal spacer to make it look decent, see #11950 (comment). This causes column numbers to change in the rest of the grid.
I recommend testing on at least one other OS than OSX to be on the safe side.