portage parameter -q is (at least I think so) intended that it should, in combination with -p, not output anything if there's nothing to do. e.g. emerge -puDq world is perfectly usable for a cron-job, that should inform admins about pending updates. for --depclean, something similar is not possible. I'd expect emerge -pq --depclean not to produce output if there's nothing to clean. But instead it produces a big warning (which is pointless if there's nothing to do) and a summary.
The big fat warning has already been removed for bug #184687. I'll go ahead and remove the summary too.
This has been released in 2.1.3.10.
Created attachment 132064 [details, diff] do not print a summary if --quiet and there is nothing to clean Apparently I forgot to merge this patch before releasing 2.1.3.10.
Reopening until this is released.
This has been released in 2.1.3.11.