Google will require all devices launched after January 31, 2020 to run Android 10
Android 10 was launched in early September and Google has just confirmed that all phones launched after January 31, 2020, must have 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.
Google Mobile Services in action
The step, made by the Mountain View company, 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 builds must be approved before January 31, meaning it's 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, including Google Play Store and Services that all phones outside of China come with. One major exception is the Mate 30 series, which is launching across the globe without them due to the Huawei US trade ban.