I'm a Zulip fan (and poster of this story), so you can take this with a grain of salt if you want.
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.
Ok I'm not sure I understand the issue here. I'm not an Android whiz but I thought FCM was inherently invasive, so if I ran Zulip from a security and privacy perspective, I'd want to self-host it including running my own notification server. It doesn't bother me at all if Zulip charges people to use their hosted plans, and I'm suspicious of the existence of a gratis one ("if you're not the customer...").
I guess though that an openpush/unifiedpush(?) client is another thing to install on the phone though. I probably care more about the web version than the mobile apps. I've been using nextcloud talk but it's kind of awful.
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-...