Paramount+ (formerly CBS All Access)? As far as I know it still offers two subscription modes, ad and ad-free, and subscriber stats on Wikipedia seem to be trending in a positive direction.
But I think the real issue is this: aside from services that use ads as an explicit differentiator, there is really no reason for a service that charges customers not to also add ads. It's just extra revenue. The only reason I could see for services not wanting to do this is when they want to specifically cultivate a "premium" feel. Cable in the US was/is a classic example: people paid inordinate feels for access, and then also put up with ads on top of it all. And for a large segment of the population, cable was so ubiquitous as to be unthinkable not to subscribe to it.
https://en.wikipedia.org/wiki/Paramount%2B
But I think the real issue is this: aside from services that use ads as an explicit differentiator, there is really no reason for a service that charges customers not to also add ads. It's just extra revenue. The only reason I could see for services not wanting to do this is when they want to specifically cultivate a "premium" feel. Cable in the US was/is a classic example: people paid inordinate feels for access, and then also put up with ads on top of it all. And for a large segment of the population, cable was so ubiquitous as to be unthinkable not to subscribe to it.