It would appear that the situation has deteriorated further. dpkg 1.21.2 now issues a warning on all merged-usr systems:
Setting up dpkg (1.21.2) ... dpkg: warning: System unsupported due to merged-usr-via-aliased-dirs. dpkg: warning: See <https://wiki.debian.org/Teams/Dpkg/FAQ#broken-usrmerge>. This escalation seems in direct contradiction to the tech-ctte decision in 994388. Moreover, this seems to effectively use package maintainer scripts as a means of directing a complaint at Debian users that has not gotten traction in other forums, and then directing such users at a wiki page that contradicts a prior project decision. This does not even seem to be calling for help in any meaningful way. For instance, soliciting help updating dpkg to handle such configurations might have been more productive; that still wouldn't be appropriate in a maintainer script, but it might have been productive in a mail to -devel. But this isn't soliciting help, it's just incorrectly declaring the user's system broken. This seems counterproductive and harmful.