Comment by MrJohz

But isn't the problem with X- headers that if they ever get standardised, they necessarily create this smuggling issue? Whereas if you start with an unprefixed header and standardise it under the same name, you avoid this issue.

You could also solve the problem by standardising the header with the X- prefix, but this is more confusing to users and violates the idea that X- always means "not standardised", at which point the prefix is useless anyway.