Oh, yes, the last thing in the runit changelog is:

runit (2.1.2-9.2) unstable; urgency=medium

   * non-maintainer upload
   * re-add /sbin/runit{,-init} to runit package so it remains possible to
     use runit as PID 1

-- Daniel Kahn Gillmor <dkg@???> Wed, 31 May 2017 12:44:38 -0400

So the runit-init package no longer exists, but if you want to use runit
as init you can do it manually.