Unlike the iPhone, which made the switch from LCD to OLED only a couple of years ago, the Apple Watch featured an OLED screen from the first generation model.
According to a report from industry insiders Apple will announce three iPhones in 2020 with new screen sizes and all using OLED panels. While shipments aren't necessarily the same as sales (although Apple is generally good at forecasting), this sounds like it could be another year-on-year decline in overall iPhone business.
In addition to dropping the LCD model, the report claims there will also be some drastic size changes.
One interesting though - 2018 is the iPhone's incremental "S" year and 2019 is expected to bring the new iPhone 11, which would mean that Apple will bring a complete redesign of its iPhone lineup in 2020, which would normally be an "S" year.
The report is based on sources from Apple's component supply chain, and they add that the company is still unsure whether to stick to Samsung-provided displays or defect to LG for its touch technology. In fact, the largest of next year's models will be almost 0.6 inches bigger than the XS Max, while the smallest model will be about the same size as the 8 Plus. This should reduce display costs, suggesting the smaller iPhone will be the new budget model.