My garden doesn't have a fence, doesn't mean you can host your picnic here.

Wikimedia has a User-Agent policy which is being violated here. Hence this is the property owner putting up a sign that says "risk of injury", so if you walk in and injure yourself, you only have to blame yourself for being negligent.

The policy is for how wikipedia will act when encountering clients with certain user-agent headers, not a rule for the clients.

It's a policy how wikimedia acts when clients lack a user agent header, it's therefore effectively a rule for clients as without a proper UA header, they may be blocked indefinitely.

Is this something real (in US, most probably)?

Yes, you can sue anyone for anything. Your suit probably won't prevail, unless you have access to very expensive lawyers and your opponent doesn't.

But you can totally sue anyone for anything, and that makes for entertaining headlines - even though if plaintiff lost promptly

The problem of course is that the "victim" has a lawyer operating on a contingency, whereas you have to pay your legal costs, and generally cannot recuperate them.

In France (at least), all swimming pools are protected by a fence. If you own a pool and don't put a fence around it, you can be held responsible for a child drowning into it.

It is possible this principle applies to other countries and other things than pools.

Here in Russia, if you leave poisonous chemicals like methanol, etc, unmarked or put a bear trap in your locked house behind a locked fence with a generic warning sign, and then someone dies or gets injured by these, chances are you will go to jail. Idk if this applies to accidental traps like pools or rakes in grass. Same for taking a knife out of an attackers hand and stabbing them back. (Yes, our laws protect criminals better than citizens, not joking.)

Interesting. So if I understand this correctly, if someone breaks into your house and gets injured, and they can make a good case for some kind of negligence on your part, then they can successfully sue you?

In Poland setting marked traps on your own, fenced property is illegal and their owner is responsible for any harm they cause, because there exist legal reasons to enter another person's property - for example to fight spreading fire.

However my favourite example is the law that allows any bee keeper to enter any private property if they are pursuing fleeing bee swarm.

Leaving a bear trap goes way beyond negligence, it's literally setting a trap. Similar with unmarked dangerous chemicals, they're required to be marked for good reason.

In Greece if a burglar dies while in your house you can be held responsible, even more so if you have set up traps.

If a judge or an expert is sure that you intended this outcome, and that someone is brave (or dead) enough to admit their own crime.

It's also illegal to set a trap in your own home in the US as well, decided when a property owner, tired of people breaking into his property while he was away, set up a shotgun booby trap that injured a burglar. https://youtu.be/bV9ppvY8Nx4

I wasn't sure if it is the same or similar principle in Russia or a different one that requires active care for a burglar. Unlabeled chemicals causing liability for a burglar seems extreme to me

This is an urban legend in Russia.

Only in your dreams and some dumb countries, not in the rest of the world.

You think this, but how much experience do you have with it? People know that homeowners have insurance. They sue to make the insurance pay out. It happened to my neighbor. So you can make all of the dumb countries comments you want, but it doesn't make it any less real.