I've been paid to (help) maintain an open source project by a company that also offer services and support around that project. When someone report a bug on GitHub, then the bug is triaged and prioritized for both paying customer and open source users. But sometimes open source users would just ask questions in the GitHub bug tracker. Then I'd just close the issue and provide a link to the official support channels (sometimes also answering the question if that's easy enough, but not if I have to ask more info or if I have to search for the answer). Same when open source users would just ask questions by email.