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All I gave them are a private relay email address. So I am assuming there isn’t much else in terms of private data.
I am sure your ip address is logged somewhere for security. But beyond that I am not sure what else is there to store.
There's tons more data than that that can be picked up. To start with, what posts you interact with and how. I'm sure there's loads of other data points that can be tracked.
Thankfully the code is open source so a good answer can be had. Unfortunately not familiar with the code base
Ofc instances themselves may modify the code to track certain things. However this highlights the importance of using instances you trust.
I wonder if there is a checksum or something to verify what version of code an instance is using
True, open source is always better for the sake of transparency. Worth noting, though, that doesn't mean the server you are browsing on uses the exact build they say they are.