Xiaomi is adopting a new policy for the delivery of beta MIUI ROMs. Henceforth, devices that are more than a year older will be gradually withdrawn from participating in the beta program. Entry-level Redmi devices are also exempted from enrolling for MIUI beta. That being said, this does not affect the eligibility of the aforementioned devices from getting stable MIUI updates.
https://www.notebookcheck.net/Older-and-entry-level-Xiaomi-phones-will-no-longer-receive-MIUI-beta-ROMs.423684.0.html