I note the discussion of this issue in #3429667, so this is just putting on record that someone is interested in seeing the feature return.
As a sidenote, the vncviewer manpage (on Gentoo, user-submitted 1.2.0 ebuild) currently still lists the -via option. If the feature does return, I submit that the description could do with some clarification on how to correctly use it for complex use-cases, e.g. non-standard SSH or local/remote bound VNC ports, or specifying the username on the gateway host. I had to resort to a lot of googling and trial-and-error for my use-case (which combines all of the above!) ... before finally learning the feature wasn't actually implemented :/