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Virgin Media Warns UK Broadband Users Not to Switch Routers Off at Night - ISPreview UK
(www.ispreview.co.uk)
This is a most excellent place for technology news and articles.
Most providers in the US allow it too. It's great that Germany has it enshrined in law, but in practice it's not the exception.
It's been allowed everywhere I have ever lived in the US.
The issues you'll run into is they get all stupid about it if your service ever goes down. They'll always blame your router/modem first. (Literally the entire neighborhood could be down and they'll act like it's something specific to your device). Sometimes they try to charge an install fee or a connection fee or other dumb shit.
I think their are local laws that require them to allow byod too. It depends on your area though.