It is an old habit. Android smartphones are released with an older version several months after a new version already exists. In order to delimit this, Google has set a date when it does not accept pre-Android devices 10.
The measure was discovered in the Google Mobile Services documents by the XDA-Developers forum. According to this source, 31 January will be the last date that Google will approve smartphones that have Android Pie.
This means from February 1, 2020, manufacturers will only be able to submit products to Google for certification with the new Android 10. A very good measure by Google, and goes against fragmentation.
Android Pie smartphones can be released after this date
Please note that Android Pie devices may be released after January 2020. It is sufficient that they have been approved within this deadline.
These Google Mobile Services documents relate to the Google apps we know so well. We’re talking about products like the Play Store, Youtube or Gmail, which come in all Android smartphones outside of China. Or almost all of them.
Google thus seems to be fed up with the fragmentation we find in the Android system. Being an open source system has its advantages, but it takes steps like this to reduce fragmentation between versions.