I thought that tags composed only of numbers could not be created. Was this is a recent change, or am I misremembering the block?
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.