Sonar does "know" if a synth is "used" by checking for any midi track set to send to the synth track. Muted or not is a mix decision, and secondary to the question.
Many (including me) have asked for a couple of "matrix" views of routings. This view could be sorted by plugs, enabled, disabled,...
And by buses: what tracks/buses are routed to what bus(es) directly, by sends,... what buses have no input,...
Tracks: muted, soloed, EQ enabled,....
All of this stuff could be summarized on a screen for quick reference. And then for ability to make such changes.
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Many programs such as InDesign have the feature described by HelloGoodbye. 'Twould be a good addition to Sonar.
j