There is a real safety issue with plug-in solar panels and plug-in batteries. Things go wrong if other loads are on the same circuit, which is almost unavoidable with a plug-in system.

Consider a circuit in a home, designed to carry 16A like a common EU/UK circuit, protected by a 16A breaker. Then plug in solar or a battery that delivers just a small 10A. Now in case some other thing on that circuit draws 26A, the breaker doesn't stop it and the circuit is overloaded.

If that same solar was installed as a fixed setup on its own circuit with no other loads on it, it would be safe and protected by the 16A breaker in the switchboard. It's the combination with other loads that causes issues.

Plugin solar doesn't make much impact anyway even in Germany because of shades/angles and most of the time- no storage. Rooftop is another discussion

I believe it’s only legal in Utah so far in the US: they legislated it last year, and apparently half the country is expected to pass a copy-paste version in their next sessions

It hasn't changed... yet. The media noise is because the government has announced that they were reviewing current rules with the aim of allowing "balcony solar" by the end of the year.