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
"The firmware is still a nightly status, unfortunately."
Heyyo Notebook Check, err, I think clarification is needed here. Nightly builds are for the current officially supported version of Android (Which is LineageOS 17.1 based on Android 10) and weekly for a previous version (LineageOS 16.0 based on Android 9).
LineageOS only officially supports two versions of Android at a time, albeit security patches are usually backported by the community for previous versions of LineageOS for legacy devices that might not run newer builds of Android well or in the case of many devices on legacy IMS stack that must stay on LineageOS 15.1 to keep VoLTE support for their unofficial builds (IE, devices with Snapdragon 810, so 2015 devices are usually on legacy IMS stack).
Unfortunately at this time even 17.1 are temporarily on weekly builds despite the nightly build status since the build servers are still partially under maintenance due to that hack that LineageOS suffered a while back.
Thank you very much for pointing that out!