[price]

> they all rely on hosting your own server, or some kind of strictly limited "community" plan

Happily this is not true of Zulip! Open-source projects get free hosting on zulipchat.com, with the exact same features as our corporate customers. Quoting from https://zulipchat.com/for/open-source/ :

> The hosting is supported by (and is identical to) zulipchat.com's commercial offerings. This offer extends to any community involved in supporting free and open source software: development projects, foundations, meetups, hackathons, conference committees, and more.

(I work on Zulip.)