It has been just over four years since Xiaomi launched the Mi 5 and Mi 5 Plus. Since then, we have had four consecutive flagship series in the Mi 6, Mi 8, Mi 9 and Mi 10 series. However, there are people who still hold on to their Mi 5 models. But about receiving a new OS update from Xiaomi is currently not out of the question. Models originally loaded Android 6.0 Marshmallow, but have been updated to MIUI 10 based on Android 8.0 Oreo as the latest major update. 
However, if you have a Mi 5 or Mi 5 Plus and want to try out the latest Android version on your good old Mi 5 or Mi 5 Plus, Lineage OS will come to your rescue. LineageOS 18.0 ported Mi 5 to Android 11.
On the Mi 5 support page on the XDA Developers site (via 1, 2) it is argued that almost everything works even with a third-party device. Furthermore, LineageOS 18.0 supports all Mi 5 configurations. The custom ROM is in beta anyway and you should expect some bugs and glitches.
On the other hand, the Mi 5s Plus ROM, which is different from the Mi 5 ROM, is in the testing phase. Also, the Wfd service does not work with this assembly.
If you plan on porting a custom ROM to this, make sure you save all your data before installing any of these ROMs.
