I could see an argument that ads for specific, chess-related products like Chessable courses would be welcome on Lichess, perhaps even as an opt-in so users can choose to support the site. I know Chessable is constantly introducing new courses and I don't always remember to check regularly. A banner that tells me of a new course I'm interested in would solve that. For me, at least, but I doubt I'm the only one.

I wish more sites would do ads without using ad networks. Communities like the chess world are small enough that all the providers know each other and could probably work together in a way that benefits everyone, including users.