It has been a little over four years since Xiaomi launched the Mi 5 and Mi 5 Plus. We have had four successive flagship series since then in the Mi 6, Mi 8, Mi 9 and Mi 10 series. However, there are persons who still hold on to their Mi 5 models. But getting new OS update from Xiaomi is presently out of the question. The models originally boot Android 6.0 Marshmallow but were upgraded to Android 8.0 Oreo-based MIUI 10 as its final major upgrade. 
However, if you own the Mi 5 or Mi 5 Plus and would like to experience the latest Android version on your good old Mi 5 or Mi 5 Plus, Lineage OS has come to the rescue. LineageOS 18.0 has ported the Mi 5 to Android 11.
A support page for the Mi 5 at XDA Developers (via 1, 2) claims that even with the third-party arrangement, practically everything works. Also, LineageOS 18.0 supports all configurations of the Mi 5. The custom ROM is in a beta phase anyway and you should expect some bugs and glitches.
On the other hand, the Mi 5s Plus ROM, which is not the same as the one for the Mi 5, is in a testing phase. Also, the Wfd Service does not work with this build.
In case you plan on porting to this custom ROM, do ensure that you save all your data before installing either of these ROMs.
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