We are actively working on making full disk encryption work with non-amd64 platforms on UC20, but that work is scheduled for a bit later this year, more details to follow…
In the meantime for anyone interested in playing with FDE on amd64, provided you have a device that uses UEFI and has secure boot enabled and turned on with an unprovisioned TPM2.0 device, you can flash the UC20 beta images onto the device and get full disk encryption by default. See more details here: UC20 beta1 released