Strikethrough is incredibly difficult to read. If authors need or want to remove part of their text from the "main" body of the node without deleting it, I'm in favor of offering a better tool than strikethrough for that. Readmore works fine, but conveys the wrong message to the casual reader.
But the author could put a blurb just before the readmore tag explaining why it is beign used such as soemthing like "This portion no longer relevant:".
<spoiler> doesn't support this, but that is probably good that it doesn't (to discourage the increasing mis-use of <spoiler> for things other than spoilers).