What is "iTunes Device Support Upgrade"? What I know:


Component of "Software Update" (and is visible when you open the list).
Requires reboot
108 MB (for Sierra)
is hidden in the "Updates Installed in Last 30 days list
(probably) is not part of the auto-installing "Install system data and security updates" track


First appeared with "macOS Mojave 10.14.5, Security Update 2019-003 High Sierra, Security Update 2019-003 Sierra".

Tried Apple sites and some basic googling.
The iTunes Device Support Update addressed a rare issue that could cause problems with updating and/or restoring devices through iTunes on macOS. This update was not required for Windows versions of iTunes.

This update was recommended for any macOS users who use iTunes to update or restore their iOS devices. It ensures that these updates/restores work properly.

[UPDATE]

Below is a screenshot taken from the Mac App Store app running within El Capitan showing the iTunes Device Support Update and its description: