A good idea of the MIUI 13 release date has been offered up by a super moderator on the Mi Community forum. Along with that, a refreshed list of Xiaomi, Redmi, POCO, and Black Shark devices that are likely to land a MIUI 13 update has now included the POCO C3, Xiaomi Mi Mix Alpha, and the new Mi 10T range.
https://www.notebookcheck.net/MIUI-13-release-date-rumor-joins-predicted-eligibility-list-of-current-Xiaomi-smartphones-that-are-likely-to-land-an-update.506386.0.html