No. I know it was something I configured in FluffyChat, and for that to work it must have sent the configuration to the Synapse server; my account is on matrix.org. This is in the Notifications settings of FluffyChat:
Tusky (Mastodon) can also use ntfy, so Mastodon server(s) must support it too. Ring, most (all?) of the Futo communications apps, like Circles. I think ntfy is common for the fdroid/OSS app developers; I see it a lot in OSS apps, like Home Assistant.