I don’t really think we’d want to actually remove any of the old containers, certainly not for a few years. Even if we stop using them ourselves, we don’t want to break user’s builds unnecessarily.
And yes, some of them (especially OwnedArray) do things that are lacking from the std library. You could argue that Array could be replaced by std::vector (which correctly handles a few things that Array couldn’t do because it needed to work with old compilers), and OwnedArray with std::vectorstd::unique_ptr but std::vector was designed a long time ago and is a bit clunky to use IMHO.