(Sorry for the delay, I haven't checked HN is a while and just noticed this)
So some proxies (forward and reverse) will strip this header for different reasons.
The reverse proxies sometimes strip it to improve caching and/or because they were configured to have a allow-list of headers and this (being a less common one) was excluded.
The forward proxies will sometimes strip it for privacy reasons (it can be used in fingerprinting) or for the same reason (they have a list of allowed headers and this isn't one).
