VOA from US

Yes, you can. If you are a US citizen, you are eligible for 5 day Shenzhen VOA, but you have to be aware that the price for a Shenzhen VOA for US citizens is 956 CNY. The best place to obtain a VOA is Lo Wu / Luohu border crossing. It is advisable that you bring that amount in Chinese currency that you can easily obtain at one of many Hong Kong money changers.

Hello. A Chinese friend said that there is a new law that allows US passport holders to stay up to 8 days. Is this true? The law has been enacted very recently. Could you elaborate please?

This is not a correct information. At least at the time being, there is no a single initiative to change a single detail regarding Shenzhen VOA policy.