The LXD system container and virtual manager, LXC container runtime, and
LXCFS FUSE filesystem projects have released version 4.0 LTS. LTS versions
of these intertwined projects are released every 2 years and receive 5
years of security and bugfix support.

Source: LWN.net – New 4.0 LTS releases for LXD, LXC and LXCFS