Their lack of required information to dox me. The "worst" thing they have about me is my IP and what I post in this public forum, but it's not like they know my actual name or where I live or anything.
They have the times that you post. Your normal travel data. Types of devices or browsers you post from. What topics you post too. What individuals you communicate with.
I don’t know what they could do with this information, but we be shouldnt be ignorant of it.
Aside from the types of devices or browsers you post from, all that is included in the "and what I post in this public forum" part the parent comment already mentioned.
Yeah, nobody should be giving their personal information to any online services. The worst thing my instance admins could do is dox my IP (which doesn't even geolocate to the city I'm in) and my burner email.
Not giving them enough information to be able to do so in the first place. In fact, this works to thwart doxxing anywhere for most people. Exceptions would be people on static IPs, who's IP address may resolve to their direct home address. This isn't common, though.
Not a ton. Loss of community reputation, legal stuff, that's about it. A good software developer does not necessarily a good administrator make.
And if the thought of being doxxed is a concern, consider data breaches and law enforcement requests too. These may happen even with a benevolent administrator.
I think you've just asked the question "Why doesn't the US have information privacy laws?"
In any case, if an instance owner did this and they had at least one user from the EU, they'd get sued into oblivion. Meta, Apple, and Google constantly get hundreds of millions of dollars in fines over stuff like this.
Nothing. Don't say or do anything on instances ran by people you don't trust.
If privacy is that important to you though, set up an email address for your "online" aliases so it can't tie you to your real name. Also if you are really worried about IP addresses leaking (IMO, you probably shouldn't be), just use a VPN. And don't send people PMs that you don't want to end up having to justify to a lawyer.
Also note that I can't think of anything that would kill an instance faster than their admin leaking email addresses and PMs. Most people are probably not important enough to risk killing an entire instance over.
Law. They are paying their housing provider via some bank account. For the police, that means they are easy to find. However, if they pay with crypto for hosting, or use some other way to hide from the law, they may be safe.