There were some issues with specific kernel features not being upstreamed timely.
Having said that, here is a distribution usage for the snap package of LXD, https://snapcraft.io/lxd (see that the end of the page).
Having said that, here is a distribution usage for the snap package of LXD, https://snapcraft.io/lxd (see that the end of the page).
The other big one for a long time was fuse inside user namespaces, but that has been merged upstream in 4.18.
Some of the AppArmor features were also a sticky point for a while as upstream was lagging behind quite a bit, but the AppArmor maintainer has since fixed that, so recent Linux kernels have everything that we use.