the slice module was implemented in 1.9.8, but it's missing in the actual ebuilds. See also: http://nginx.org/en/docs/http/ngx_http_slice_module.html Thx for your help!
It will be added shortly in the 1.9.x series; work in progress ebuild here: https://github.com/gentoo/gentoo/compare/master...jbergstroem:verbump/www-servers/nginx
commit 2499a68efa4f3b685de907ed6e1e17afb408058f Author: Johan Bergström <bugs@bergstroem.nu> Date: Wed Feb 3 23:08:05 2016 +0100 www-servers/nginx: Add auth_ldap, stream, slice modules This commit adds http_slice, stream, auth_ldap modules and support for libressl. It also updates 3rd party modules, enables http2 by default and disables gzip by default. Fixed in 1.9.10-r1