[underslung]

It's about consistency and intent -- these observations do not extend to conspiracy tier thinking.

What is the primary purpose of the X-Client-Data header if not for some unplanned future-proofing?

Masquerading about it does not add to credibility

[joshuamorton]

It's for tracking which chrome experiments are enabled, allowing chrome to check the impact of new chrome features by enabling them for only some populations and comparing the results. As it says in the white paper: "This header is used to evaluate the effect on Google servers - for example, a networking change may affect YouTube video load speed or an Omnibox ranking update may result in more helpful Google Search results."

[WesolyKubeczek]

That's describing it short-term at best, as some have pointed to you already.

Also, I don't see the important word "only" in the passage you're quoting, so given the usual MO of profit-driven corporations I'm just going to assume they omit other purposes of which there's at least a ton.