We described those symlinks as legacy. But they are part of the ABI,
so they are not going away. So the symlinks themselves are not legacy,
the original directories were, but we are describing the symlinks not
the old layout.

In a multiarch layout, /lib64 does not point to $libdir. It points to
a directory with loaders (or more likely symlinks to) for different
architectures. The premise of multiarch is parallel installation
of multiple architectures, so pointing to a single architecture
(eithr the primary architecture denoted by $libdir or any other)
is incompatible with that.

This document inherited the recommandation for multiarch from its
precedessor, the file-hierarchy man page. The multiarch layout _is_
elegant and flexible, but it hasn't been widely adopted, it is only used
by the Debian family of distributions. At this point, it doesn't seem
likely that it'll be adopted elsewhere. Wiring up installations in this
more complex scheme if much more effort. The ability to support multiple
architectures from a single installation was more useful when
installations were shared. Nowadays, small, tailored images are used
much more often. In particular, for immutable installations, the size of
the image creates a pretty significant cost, so installations that are
trimmed down to support just one architecture make sense. Users who want
to run something on a different architecture will probably download a
full image and spawn a VM. We shouldn't say that multiarch is the
future, or recommend it for "new" installations. New distro efforts are
not doing this and we shouldn't be trying to convince more gullible
readers to try this.