We're re-factoring some things to try to simplify Virtualmin configuration and code. QMail+LDAP support in Virtualmin is a complicated special case that has a lot of code, a lot of configuration, and behaves unlike other elements of the mail stack. If we can get rid of it, Virtualmin would get a little nicer to use (fewer configuration options) for everyone, and we'd get rid of some code complexity that we don't think is providing any value.
Note that we're not changing LDAP support for Postfix or Sendmail (and QMail with local users would continue to be supported). We know popularity of QMail, in general, has dwindled tremendously over the years, and we haven't heard much about it in a long while...so, we're hoping we can kill off support without impacting anyone.
As far as we can tell based on forum and web searches, nobody has mentioned this particular combo in about 7 years, so if somebody is using it, they're being quiet about it.
Holler if you use it, or forever hold your peace. (Also tell us what features you using that wouldn't work with Postfix, and we'll try to add them and convince you to switch, so we can get rid of this weird old code anyway.) ;-)