Keychat currently uses Google’s Firebase Cloud Messaging (FCM) service for message notifications. If you turn off notifications, this service will no longer be used.
nostr:nprofile1qqsth7fr42fyvpjl3rzqclvm7cwves8l8l8lqedgevhlfnamvgyg78spzpmhxue69uhkummnw3ezumrpdejqz9rhwden5te0wfjkccte9ejxzmt4wvhxjmcpp4mhxue69uhkummn9ekx7mqqdtrej - why does the app try to connect to Google?
Installed from nostr:nprofile1qqs83nn04fezvsu89p8xg7axjwye2u67errat3dx2um725fs7qnrqlgzqtdq0

Discussion
Does that mean receiving addresses are known to Google, by default?

Yes.
The core reason chat apps need push notifications is that mobile OSes don’t allow apps to stay active in the background. Without push, the app can’t receive messages or notify users when it’s not open.
Notification services have become increasingly centralized, and we’re deeply dissatisfied with that. It’s up to each user to choose whether to enable or disable message notifications.