The inservariables plugin was provided by personalization/MCM, this never got ported to Touch UI (as the team has moved to Adobe Campaign from AEM/CQ MCM implementation).
There is a Touch UI-compatible version of the Adobe Campaign insertvariable plugin (which gets the variables from a AC instance), so you could use this as a starting point for creating a custom Touch UI-based version of the original insertvariables plugin.
Your entry point would be /libs/mcm/campaign/components/touch-ui/clientlibs/rte/personalizationplugin/plugin.js