I think the latest change from "instance" to "server" is not appropriate. The server may contain many instances, and the admin might not have anything to do with a software installed on the server. So a report sent to server moderators might yield no results at all: what you need is communicating with the people managing the Mastondon installation you're interested in, not the server admins.

Fosstodon

Fosstodon is a Mastodon instance that is open to anyone who is interested in technology; particularly free & open source software.