I've just posted an article on Reddit, this follows some investigation I've done over the past few weeks.
Basis of the reddit post is, regardless of whether you have purchased a Sim free handset or not the Google Services Framework is identifying handsets by Carrier and only releasing the update if the carrier has approved it.
I've been able to test this theory in the UK, I'm on EE, I've been checking for updates every few days but it hasn't happened, over the weekend a reddit user said that they swapped their EE sim for a Vodafone SIM and it received Oreo. I tried this out today and within a minute I received the Oreo update!
This is extremely disappointing as so many will have gone Sim free to bypass this type of carrier controlled approved update process.
Thats how it should work, Nokia need to be more transparent about the update process.