The Deck was such a thing. It was sort of invite only because once you go first party you have no way to validate the user base so you need to trust the partner. For ads that result in direct sales this can be easy to do though.

It was more of an ad network, no? Also I think it shut down.

I'm talking about something even simpler than that. I have my own website and I have my own advertisers who want to put ads on it. I need a way to serve them and do contextual targeting (e.g. stories about a certain topic) and frequency capping and forecasting and the other sort of basic stuff I expect from Google Ad Manager.