https://gottcode.org/xfce4-whiskermenu-plugin/ is at 2.0.1 and supports gtk 3.18 as the minimum. There are other improvements from 1.6 upward.
Thanks for the report. I filed a PR. https://github.com/gentoo/gentoo/pull/2132
commit 92dd860deaa96fd46e14244854103fa3f2e4a89d Author: Christian Tietz <christian.tietz@mailbox.org> AuthorDate: Wed Aug 24 00:03:48 2016 +0200 Commit: Patrice Clement <monsieurp@gentoo.org> CommitDate: Tue Aug 30 20:31:11 2016 +0200 xfce-extra/xfce4-whiskermenu-plugin: version bump to 2.0.1. Package-Manager: portage-2.2.28 Closes: https://github.com/gentoo/gentoo/pull/2132 Signed-off-by: Patrice Clement <monsieurp@gentoo.org>