I thought that tags composed only of numbers could not be created. Was this is a recent change, or am I misremembering the block?
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I'm pretty sure you're misremembering.
Tags with numbers in them are sometimes interpreted as version tags, which has some flaws, but other than that... if the tag itself is just a number because that's the concept it refers to, that seems fine to me.
Perhaps "http-400" might be a better choice in this case, but if I were asking question, I'd probably just search for the number "400" when typing in a tag anyway.