It's a new property from CSS Display module level 3.
See: https://rachelandrew.co.uk/archives/2016/07/22/the-proposed-box-suppress-property

TLDR:
box-suppress: show is the default.
box-suppress: discard is the same as display: none but without tinkering the display value before it was hidden, which nowadays can be so many things.
box-suppress: hide is synonymous to visibility: hidden, as far as I can make out.