It is not used within ebuild.
commit 3224b2e4d149a973cbc38f066c17efc2178b9a63 Author: Matthias Maier <tamiko@gentoo.org> Date: Tue Apr 25 08:10:30 2017 -0500 app-emulation/qemu: remove unused use flag "threads", bug #615878 Package-Manager: Portage-2.3.5, Repoman-2.3.2