An interesting, though old, read here: https://zerocredibility.wordpress.com/2009/08/24/why-android-swap-doesnt-make-sense/
Main points why SWAP does not make sense in Android:
1. In Desktop environment, swap space is used when the OS is running out of physical memory and it decides to use a small of part of HDD to act as slow memory. In desktop, there are many ideal processes and the eviction policy to swap will move their pages to swap. This results in marginally degraded performance which users hardly notice. However, in Android, most of the processes are active and hence the user will notice the difference.
2. SD card are very very slow (https://blog.hellotech.com/portable-hard-drives-flash-ssd-sd/) compared to HDD and hence the SWAP performance wil be very poor.
So, in Android if the OS runs out of memory it just kills the process. However, it notifies the process that it is about to be killed (using callback methods) and it should save important data to persistent memory.