Is there a way to get list of unused plugins across the environment?
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No, but you can check for any dependencies for each plugin in the admin console.
Which version of Appian allows this? I'm using 21.4, and see no option for that
Not sure. Positive on 23.3 & 23.4. Please check the release notes.
Per the 21.2 Release Notes, it was added in 20.3: