The new router system breaks the ability to have '/' in an autocomplete_path. As such, it's been suggested that we change the parameters passed to autocomplete as a GET string instead.
This issue was discovered in #1801356: Entity reference autocomplete using routes.
Also, is being discussed in a more general sense in #1827544: [Discussion] Drop automated passing of extra argument: Y/N.
Modify Form API #autocomplete_path to pass parameters in a GET string instead.
User interface changes
All modules implementing autocomplete_path callbacks would need to modified to use new syntax.