Remove non essential classes from core - the classes are in classy, cores templates should be as clean as possible.
js required classes should be prefixed to js- according to code standards
See parent issue(s) and related issue(s) for details,
Twig Templates to modify
core/modules/system/templates/status-report.html.twig core/modules/system/templates/system-admin-index.html.twig core/modules/system/templates/system-config-form.html.twig core/modules/system/templates/system-themes-page.html.twig
Do not modify templates with a strike through their name. Those were not copied to Classy.