5.5.4 had a critical flaw in it, so it was pulled to prevent people moving to a broken version.
It's a black and white flaw though - if you haven't hit it by now, you probably won't on your current platform. You could ask https://support.atlassian.com if they'd provide you with a copy, as you probably aren't affected by it, but I'd be looking at moving to 5.5.6+ (5.5.5 was affected too I think)
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