what happens if someone posts their private key there.
That's like the first rule of Bitcoin. You break it, you lose it.
If someone is too childish to post their private key which has balance or has plan to use the address in the future then they do not deserve bitcoin. Let them be lol
Some people do not just know much about what private key his, only when their Bitcoin has been lost will they remember they have been fooled, although that is being a fool truly. Some ways wallet generate seed phrase and private may not include such warning like 'revealing the seed phrase or private key to anyone can be used to steal the whole Bitcoin the seed or private key control from the owner', this kind of information might make the foolish newbies to avoid such scam. But, you are not wrong, anyone that reveal his/her seed phrase are not worthy to have Bitcoin.