According to
https://support.motorola.com/uk/en/softwareupgrade, my Moto X Play will remain on 7.1.1. So it's obviously a good job that I've been on LineageOS for most of this year, as that was already at 7.1.2, and I keep upgrading to the weekly "nightlies" available for it.
Whether 15.1 or newer will become available for my model however is another question. A search indicates an unofficial LineageOS build was being worked on for my model, but there was said to be at least one part that doesn't work in the available builds and it hasn't been updated in a number of months now.
At least the nightlies keep me up to date on security patches.
It seems however that if I want a more substantial upgrade in the software sense, it would mean a more substantial upgrade in the hardware sense too. I'll have to see if something catches my eye, but it's a shame as it has only been two years from purchase. At least it has been just as fast this whole time, and assuming the LineageOS 14.1 builds keep coming in the meantime, I can continue to wait happily enough until I find something that I feel is worth it.