There are two types of official packages for LXD, the deb packages and the snap packages.
Ubuntu 16.04 LTS and Ubuntu 18.04 LTS come preinstalled with LXD as a deb package, and currently are the only distribution versions that have deb packages of LXD.
- Ubuntu 16.04 LTS has the deb package of LXD 2.0.x (supported until 2016+5years = 2021).
- Ubuntu 18.04 LTS has the deb package of LXD 3.0.x (supported until 2018+5years = 2023).
All other distributions and distro versions get the snap package of LXD, currently at version 3.0. It is possible to switch (migrate) an installation of Ubuntu 16.04 or 18.04 to the snap package of LXD.
The snap package of LXD is an official package, and is available from the Ubuntu Snap Store. Snap packages have a facility to autoupdate. There is a discussion at the snap development forum on how to deal with autoupdates and whether some packages can get a reprieve from autoupdating. That discussion has not been finalized, and the main idea is that any decision that is taken, it should be generic enough to apply to all snap packages. When the LXD snap autoupdates, it should normally not restart the containers.
At some point the discussion will conclude with some good solution.
In any case, run
snap info lxd to see the available channels and the choice of LXD packages.
For your case though, you are fine to stick to the LXD version of your Ubuntu LTS.
If you want to migrate to the snap package, there is an option to migrate to the LXD 3.0.x stable snap, or the LXD 3.1.x stable snap (the one with all the good new features).