1. Regarding "minimal costs" of notifications, the project lead said in an HN comment (https://news.ycombinator.com/item?id=38661960) from when this was announced, "The problem statement is not that we need to fund the costs of delivering push notifications. We need to fund the costs of building Zulip -- the server, apps for every platform, support for a vast range of different self-hosted configurations, etc." That was on the announcement thread on HN, so you can find more from him and other commenters there.
2. They offer free plans (for the app/service itself, and also just notifications if you're self-hosted) for lots of non-commercial organizations (including open source projects and communities). Discounts for larger non-profits and others. The info is prominent on their pricing page: https://zulip.com/plans/
3. There's a discounted plan for just mobile notifications.
Here's their original announcement post, with an update linked to from the top: https://blog.zulip.com/2023/12/15/new-plans-for-self-hosted-...