I searched over the web for news about official Miui ROMs released by Xiaomi for Mi2s, but I couldn't find anything newer than Lollipop. Is the development of new Miui ROMs for Mi2s officially dismissed?
How you mean development officially dismissed? It gets updated every friday. Seems like proper support to me. Android version is not everything you know, it's probably stuck at android 5.0 now unless you flash a custom rom. It will even get official MIUI8 so no reason to complain when it comes to support.