As I understand the situation, this capability has been discarded from LXD.
There are valid use cases for being able to host a container containing a foreign arch rootfs using the user-space QEMU interpreter shims.
At Camgian Microsystems, we develop software targeting armhf and other non x86 family processors.
In our build pipelines, we currently use Docker to host cross-target containers with foreign arch root filesystems for executing various quality-assurance tests. There are other very useful tactics we employ that help us deal with certain softwares that are not set up for cross-compilation, etc.
We really like the functionality and feature set of LXD and desire to migrate to it but the lack of equivalent binfmt support is presently a deal-breaker for our shop.
Performance attributes and lack of ptrace support are not primary considerations for our use cases.