slurs across voices

[Jim Cline]

I would like to create slurs between alternating notes across two voices 
in this example:

\version "2.20.0"
lower =
  \relative c {
   \time 3/4
  \clef bass
  <<{\stemUp e''16\rest cis4 bis ais8~ais16}\\{\clef treble\stemDown 
\slurDown e4 dis cis}>>|
}
\score {
    \new Staff = "lower" \lower

The documentation provides an example in 
http://lilypond.org/doc/v2.21/Documentation/snippets/expressive-marks 
("creating slurs across voices") but it only works in the case where beats 
are missing in each voice.  In my case, each voice has the full number of
beats to make a measure.  It seems like this issue was last discussed in 
2015, 
https://lists.gnu.org/archive/html/lilypond-devel/2015-03/msg00288.html
unless I missed something.  Has this limitation been addressed since then?

regards, Jim