After a long wait, the Android 10-based LineageOS 17.1 ROM is available for the Xiaomi Mi A1. The firmware is still a nightly status, unfortunately. Other handsets that got Android 10 support thanks to LineageOS 17.1 in the last few days include the OnePlus 5/5T and later models, LG V20, Honor View 10, HTC One 2014, and others.
https://www.notebookcheck.net/Android-10-based-LineageOS-17-1-is-finally-available-for-the-Xiaomi-Mi-A1-and-other-handsets.478784.0.html