Apparently, when you want to support 1.0, you still have to use the old approach. And as the old approach still works in newer languages, it hence seems to be the recommended approach to stick to the old. That is a pity, as I really appreciate the new setup with a new Project file under the test directory.
From my perspective it seems like adding the support for the project file to 1.0 is possible in principle, as this does not break existing supported setups. Of course if someone used a test/Project.toml for something unrelated that would break, but I think that is not a big issue as it was not supported anyway.
Hence I would argue for including the test/Project.toml support in the next 1.0.X release.
This would enable everyone to use the new format without dropping support on 1.0 or having to maintain two locations of test-dependencies. Hence it could truly become the new recommended approach.
The exact interaction between Project.toml , test/Project.toml and their corresponding Manifest.toml s 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.