Problem/Motivation
In #3100937: Update modernizr to 3.8.0, it was discovered that Modernizr's touchevent test is deprecated. https://github.com/Modernizr/Modernizr/pull/2432
There are also problems with that test from Modernizr 3.7.1 onward, which requires an override of that test in Drupal core (added in the aformentioned issue).
Core's custom touchevent, added in #3100937 currently works fine, but may become outdated as newer browser versions are released and different ways of detecting touchevent become available.
Proposed resolution
Determine if there is another library capable of detecting touchevent in a way that is useful to core. If so, add that library and deprecate Modernizr touchevent.
If there isn't another library that fits that exact need, determine if it's better to support Drupal core's custom touchevent test, or using a different library that may require refactoring how touchevent/no-touchevent classes are applied and what CSS rules are based on them.