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I would like to start by saying that this is purely speculation and educated inferences based on how events have transpired with ZTE over the last year. This is not fact, but I do believe in what I am about to write.
Contrary to what users like "ggreg" have stated, I believe with great certainty that the A2017U will receive Android Oreo sometime between December 25th and December 31st. Here is why:
As noted by "Im_Axion" on reddit, a ZTE representative stated that an update is slated for the end of December. It was not stated that it was Oreo directly, but given the fact that the A2017G has already received the December security patch but the 2017U, which in the past has gotten updates before its global counterpart has, has not, I think its reasonable to assume that a larger update with more kinks to iron out is in the works.
It has been confirmed that the A2017G will receive Oreo in April 2018. That would be 4 months after the A2017U. This timeframe lines up well with the gap between the two with the Nougat update. The A2017U received Nougat in mid-March whereas the A2017G received Nougat on June 29th, basically the start of July for most users. If a pattern can be noted, it seems that ZTE released the major OS upgrades first to the Chinese model, followed by the American model about a month later and then the European model about 4 months after that.
Lastly, it is worth noting that ZTE often remains hush-hush about their phones and updates. I personally didn't expect nor anticipate ZTE upgrading the Axon 7 from 7.0 to 7.1.1, or including improvements and new features like night light, quick toggle api support and the double power button press to launch the camera. (it's gone in the latest build for some reason). It stands to reason that they will continue this sort of support until the proper Axon 7 successor. I imagine they would want to keep the costumers they have happy since they are trying to succeed where OEMs like Huawei are failing, especially in terms of breaking into the U.S. market.
Anyway, this was just my two cents. I eagerly await the Oreo update so I can apply system-wide themes from the stock ROM without root, assuming they didn't remove that feature.
On a side note, if I am correct, does anyone else think it's kind of silly that ZTE releases some updates at the end of the month, almost as if to say they're trying to be timely when realistically they come a bit late for what they are? I imagine they'd use an argument like: "we gave A2017G users the June Security patch in June," even though it was released on the 29th, or "the August Security Patch in August for American users" even though it was August 31st, a few days before the September patch could be released. I'm not complaining, just observing,