Welcome to the Community!
Here you can do this.
I hope that helps.
I know, it's an old question, but for reference: @Monika Rani's answer is the correct one. To work around the result limits, you could do multiple queries by project or some combination of other attributes.
Also, in Jira there are technically no labels that are not used on at least one issue. However, you might see the a label in the suggestions even that it has been removed from all issues. This happens every so often and typically is due to some in-browser caching.
If you are on Jira Cloud instead of Server, you can use Jira Expressions to list all labels on your instance.
Finally, there's an app for that. In fact, multiple apps are available to manage labels. If you're on Cloud, I'll give my totally biased recommendation and suggest Project Labels. (Disclaimer: I work for Polymetis Apps)
Anyway, hope that helps,