Is there a problem with ISO3166 denoted information in general or is there a specific US issue here? I would think ISO code denoted tzdata was a public good in some sense.
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Config defaults somewhere still using them? Man page examples? Tutorials using them? Or just force of habit?
Because of a lack of things compelling people to change them until it causes a breakage. And then when it does cause a breakage, most people would rather move heaven and earth to complain, research workarounds, etc. rather than just change it. (Institutional structures can also make "just" changing it far harder than that should be.)