Absolutely! If someone were searching for information about Craft CMS development, I sure would like them to find this site.
If third-party plugins are an integral part of the Craft CMS landscape, then certainly they should be on topic here. As a matter of fact, the developers of these products and services tend to have communities of their own. You should actively embrace and encourage them become part of the ecosystem of this site.
We get a lot of requests from project teams about how they can use Stack Exchange to support their communities. I see no reason why this site couldn't provide fantastic technical support for these communities.