Note the:
The exact interaction between
Project.toml,test/Project.tomland their correspondingManifest.tomls are not fully worked out, and may be subject to change in future versions. The old method of adding test-specific dependencies, described in the next section, will therefore be supported throughout all Julia 1.X releases.
Features are not backported. If you need to support LTS, then just wait for the next LTS where it will be included.