I don't think that the man page for this command is online.

This is a deprecated command, provided during the transition from
Tk 3 to Tk 8.  '''Some day''' this is going away - perhaps sooner
than later.  One should work towards rewriting code which uses
it.

----
'''DGP''' I think the comments above could be misleading.
There is no command in Tk called [[pack-old]].  Never has been.
There is a command in Tk called [[ [pack] ]].  [[ [pack] ]]
has several supported and documented subcommands that have
been the same since Tk 4.0. See

http://www.purl.org/tcl/home/man/tcl8.4/TkCmd/pack.html

Back in the days of Tk 3, [[ [pack] ]] had a different set
of subcommands.  Those subcommands are no longer part of
the official interface of [[ [pack] ]], but they do still
work.  So if you have some ancient script that ran under
Tk 3 and used those old subcommands, it's possible that it
might still work in a current wish.

No one writing code should use any of those old subcommands
of [[ [pack] ]] anymore.  But someone might notice that
[[ pack after .w ]] actually tries to do something other
than return an error, so the old subcommands of [[ [pack] ]]
are still
documented in another man page.  That man page can't be
called pack.n, so it is called pack-old.n.