Hi Karol,
According to the change log:
The whiteout grob property and \whiteout markup command
now create a white background built from multiple displaced copies of
the glyph in order to approximate the contours of its outline.
The previous rounded box white background can be achieved with the
new whiteout-box grob property and \whiteout-box markup
command. Setting the whiteout property to a number now sets
the thickness of the white outline as a multiple of staff-line
thickness. Similarly, overriding the thickness property of
the \whiteout markup command sets the thickness of the white
outline it produces.
Andrew