According the R-ints the only current uses of the `truelength` meta datum is for environment hash tables. Jim Hester just made me aware that R3.4.0 introduces a new use case: growable vectors.
I attach a patch to the R-ints manual that reflects this change. The wording is obviously just a suggestion. Additionally, it may be worth moving the footnote into the main body of the document now that there are more use cases.