If a button creating spoiler space could be added to the board, wouldn't it make more sense to create a button which produces a real spoiler prevention ('setting same text color and background color' or 'creating a box whose contents are only displayed after one clicks on the warning' seem to be the two most common spoiler prevention systems in use).
With just spoiler space you'd also need spoiler space after the spoiler and some people are fast enough readers, that they inadvertently even read the spoiler while scrolling past it.
