ipvlan support is now available in LXD 3.13, currently only from the edge channel.
The reason being that it requires extra features in liblxc, and only the edge channel at the moment has a recent enough liblxc version.
See the announcement and the further discussion in the comments at LXD 3.13 has been released
Note that the edge channel is the channel that gets updated everyday, so if you switch to that, you are living on the bleeding edge (or, you have bug reporting privileges 
).