Would getting this retrofitted mean the car has been modified and has to be declared to insurance companies as such?
In theory then absolutely yes, at an extreme you could say an insurance company might consider the increased cost of replacement parts as worthy of extra premium or they could say anything retrofitted might have other potential issues such as non factory wiring being an increased fire risk.
In practice, it’s a “standard” VW part and unlikely an insurance company assessor would bat an eyelid at it if you’d been rear ended.
Unless you get a real jobsworth who goes through the factory build sticker in the boot.
In which case “it must’ve been a misbuild at the factory mate”