Android 10 was launched in early September, 2019 and Google has just confirmed that all phones launched after January 31, 2020 must run on the new OS. According to XDA Developers, Google will authorize the use of GMS (Google Mobile Services) only if the OEM has provided a build based on the latest Android 10 OS.
The step made by Google is to ensure all manufacturers will provide up-to-date software to their users and Google will be able to issue timely security updates. However, the documentation says that builds must be approved before January 31, meaning it is technically possible for a phone to launch after that date with an older Android as long as its software was ready earlier.
The GMS is basically the Google apps package, which includes Google Play Store and Services that all phones outside of China come with. One major exception is the Huawei Mate 30 series, which is launching across the world without them due to the Huawei US trade ban.