As I wrote under another comment, this doesn't seem like a good reason or I must be missing something because I can selfhost Matrix or an XMPP server and get notifications for free with any client.
Whatever Zulip is offering here is not good and if these other offerings can make it free, Zulip should be able to too without charge or requiring I recompile their app.
Whatever Zulip is offering here is not good and if these other offerings can make it free, Zulip should be able to too without charge or requiring I recompile their app.