Kagi provides a very sophisticated web search service that is subscription-based (no ads). Since Proton is trying to become an alternative to the Google ecosystem, it could make sense for Kagi to fall within the Proton umbrella. Frankly, I find it a bit hard to justify subscribing to a search engine however if that service was part of a group of services like Proton, I'd be more in favour of subscribing to that whole bundle.