sys-apps/s6 includes the library libs6. It's not only used internally, but also by external packages like s6-rc, so it'd be useful if the installation of the static libs6.a could be controlled independent of USE=static.
Created attachment 427662 [details, diff] patch for sys-apps/s6 ebuild Here's a patch. I've also submitted it as a github pull request at https://github.com/gentoo/gentoo/pull/996 .
While I was at it, I've bumped the s6 ebuild to EAPI 6 and pushed that patch to the pull request, too. Feel free to ignore it if you want to do this yourself.
These have been added to the tree: 1d5e60b is for the use flag and 480552e is for the eapi 6 migration. In the future, be sure to use separate bugs. Also, since the installation files do not change, there is no need for a revbump.