Packages in extra are not put in the Manifest at all (except when they are used) and they will indeed be garbage collected.
I am kinda coming around thinking that the whole extra + target thing was perhaps a mistake, or at least not thought through enough for it to make sense for it to been implemented, and something like PSA: Use a project for building your docs (but for testing) should be used instead.
Using that means the testing environment is fully reproducible, which is not the case right now.