For one thing, TMobile was the only one who said the S6 was getting Oreo. Samsung never confirmed this, and honestly I believe it's BS. Samsung NEVER updated their flagships beyond 2 major updates. S6 would have been the first, if ever. But as I have said, Samsung themselves NEVER said the S6 will receive an Oreo update. Only an errant report by TMobile was taken as confirmation by various sources.
Basically don't hold your breath. S6 already received to major updates, and it is highly unlikely it will receive any official update to Oreo.