International betas of MIUI may have been terminated, but they are still going for Chinese users. Xiaomi has informed them that the last such preview build based on Android Pie (9.0) will be released on August 30, 2019. This suggests that the next beta thereafter will transition to an upcoming major software upgrade.
https://www.notebookcheck.net/Xiaomi-may-switch-to-MIUI-11-betas-at-end-of-August-2019.431819.0.html