In this question Destroying a Planewalker before its ability is used, this answer https://boardgames.stackexchange.com/a/14618/2474 was deleted as spam. The answer has been replaced with:
This answer was marked as spam or offensive and is therefore not shown - you can see the revision history for details.
I could understand to delete it because of very low quality (though I personally don't agree), but I don't get why it is spam.
I'm the mod that deleted that answer. I've never noticed this spam text behaviour before, because as Jefromi's test shows, mods don't see the replacement text.
I completely agree that this answer is not spam. In fact, I didn't act on the spam flag. The post triggered a deluge of flags - 4x 'not an answer', 2x 'very low quality', 1x 'spam', and 1x 'invalid flags'. From a moderator point of view, all of these are bundled together, and appear as a single "flagged post" that needs attention.
When a post has multiple flags, I go and look at the full context of the page to decide what to do. Although not spam, I agreed that this was so useless that it could be deleted. A side-effect of taking action by hitting 'delete' is that all the flags are automatically marked as 'helpful', and, presumably because of the presence of the spam flag, the spam-hiding text was introduced.
In other words:
Speaking as a user here and a mod on another site, I agree - the spam flag was misused on that post, and I've seen it on other deleted posts as well.
The spam flag is meant for actual spam: people trying to advertise. (And the offensive flag is for actually offensive things - hate speech and so on.) That's why the deletion with those flags triggers that replacement text, to keep people from ever even seeing spam links or offensive content. The feature was added in October. It's not for hiding away silly/stupid/pointless/misguided non-answers; it doesn't trigger for the "not an answer" flag.
So that answer? Not an answer. Whoever flagged it as spam was misusing the flag, and the text should not have been replaced.
Unfortunately I think this is something only mods can address. If the answer is undeleted, the replacement text goes away, then it can be deleted again. They can also decline incorrect flags; if the spam flags aren't marked as helpful, this feature won't be triggered.
(If you happen to be a mod, you can find deleted answers by searching for "is:answer deleted:yes" - I think you might find a few more this has happened to. But as I just discovered, mods don't actually see the replacement text. You'll have to just trust that it happens to anything where there's a spam flag showing as helpful in the post history.)
