Xiaomi has announced that its software division will stop releasing global betas of its OS, MIUI, on July 1, 2019. This applies not merely to older and slower devices, but to all phones of its brand. However, the OEM has also stated that this decision will not affect the frequency of security-related updates.
https://www.notebookcheck.net/Xiaomi-announces-Global-MIUI-Beta-Program-termination-for-all-its-phones.424130.0.html