Motorola isn't the best at updating its devices these days. Custom ROMs can often be helpful for devices without speedy official updates, but the lack of a recent kernel source code can make ROMs less stable/functional. Thankfully Motorola has released the kernel code for the original Moto Z Play's Android 7.1.1 update.
The company is a bit late on this; kernel source code is supposed to be released shortly after the Android update, but the original Z Play started to get Android 7.1.1 in June. Better late than never, I guess. Developers can find the source code on GitHub at the source link below.