There are a number of unpatched vulnerabilities in 10.11.6 that got patched in 10.12.x and 10.13.x. If there's no documentation that this is patched in 10.11.6, and there appears to be none, you should assume it's not.
If Apple haven't already stopped supprting OS X El Capitan, they will be very soon. It's around the three year mark when they stop issuing security updates. You'll need to start considering an upgrade to macOS Sierra or High Sierra or buying new hardware if your Mac doesn't support it.