OS X El Capitan Security Update 2018-001 is, I think, the most substantial update to 10.11.6 since 2016, which appears to go beyond the short list of security issues which it fixes. However, it also includes improvements to Mail, and many of the frameworks on which most apps depend. It is around 860 MB in size, expanding to over 2 GB in installable content.

Most importantly, it brings EFI firmware updates for many, bringing all but two models up to the same firmware versions as those used by Sierra and High Sierra. The resulting firmware version numbers are listed here.

The only applications updated are Mail, Activity Monitor, Bluetooth File Exchange, Grapher, and Keychain Access.

Among the other significant updates are WebServer, RAID Utility, most kernel extensions, many frameworks and private frameworks, most of the command tools in /usr/bin, many in /usr/libexec, and many in /usr/sbin.

Apart from the security fixes, the most important parts of this update are the EFI firmware updates, which are essential for all who are not yet running a later version of macOS.

This update is available from the App Store. A standalone updater is available from here.