It's very rare that Android phones receive more than two years of major software updates. Google has even stuck to that rule in the past, with the Pixel 2 being the company's first phone with three years of promised Android updates. According to T-Mobile support pages, Samsung could be planning an Oreo update for its 2015 flagships.
Support pages for the Galaxy S6, S6 Edge, S6 Edge+, and Note 5 indicate that Oreo updates for the phones are in development. Each page has the message, "T-Mobile and the device manufacturer have agreed to support a new software update." We've reached out to Samsung to confirm if this is real.
If Oreo does indeed make its way to Samsung's 2015 flagships, I'd be very impressed. As mentioned above, almost every phone manufacturer calls it quits after two years of support, which pales in comparison to the 3-5 year update lifespan of iPhones. Still, we don't know which carriers will roll out the update, and how long away the update is. The only phone in Samsung's catalog with Oreo at the moment is the Galaxy S8.