Opera never been used but advertised as Keyword in Package.json

[Bizarrus]

Search: https://github.com/search?q=repo%3Amozilla%2Fweb-ext%20opera&type=code Package: https://github.com/mozilla/web-ext/blob/420aa78db72f6c8d10f7ae6cfa8a61be7cf43820/package.json#L58

Why Opera here is listen? Opera never been used or declared on Docs.