Why isn't "move apps to SD card" available in Xperia devices from 2018?

Google¹ stopped providing the possibility to move apps to an SD card to secure the security and the privacy in Android™ 6¹. This feature was removed from Android, but Sony Mobile continued to support it for users' convenience for some time. However, we decided not to keep our solution to move apps to an SD card for users' security and privacy. So we stopped providing this feature in the Xperia XZ1 series, XZ2 series, the Xperia XZ Premium, the Xperia XA2 series, Xperia XZ3 and newer products.

We recommend users to move photo, video and music data to an SD card to save storage space in the internal memory. To move photo, video and music files, find and tap Settings > Storage > Transfer data to SD card (where available). In phones that don't have this option, you can move files manually using the pre-installed Files app or a file manager app of your choice downloadable from Google Play Store.

¹ Google and Android are trademarks of Google LLC.