I am Jannis, I maintain Backspace. This address receives the optional daily usage report that an admin of a Backspace server can switch on. It arrives as one POST /v1/ping per day. It is off on every instance until somebody turns it on, and switching it off stops it for good.
One report carries a random id for the instance, the day, the Backspace version, and counts rounded to two significant digits: registered and active users, users per client kind, spaces, channels, messages, stored megabytes, whether voice and federation are on, and how many peers the instance has. I add the country of the request as Cloudflare reports it, because the request reaches me through Cloudflare either way and the country is the one thing I would like to know that the instance itself cannot tell me.
What a report never carries: your domain, the name of your instance, any user name, any e-mail address, any message, any file name, your federation instance id, and any timestamp finer than a calendar day. I do not store the address the request came from. It is used once, as the key of the rate limiter, and never written down.
I delete every row 90 days after its day. What gets published is the aggregate over all instances, never a single instance, on the project's insights page.
The exact payload, the code that builds it and the reason each field is in it: docs/systems/telemetry.md. The source of this receiver: scripts/telemetry-receiver.