Hello @atoris
Thank you for contacting the support.
Everything which is offered in the plugin will remain free. We will not be charging anything for them.
We do not have any plans for a premium version of the plugin.
Whenever we will have them, we will announce them in our Facebook group:
https://www.facebook.com/groups/rankmathseopluginwordpress/
Hope that helps. If you have any further question(s), please let us know.
