It has been offence to negligently open a car door and cause injury to any person for years.
This amendment to the Highway Code is try to prevent further injuries or accidents by trying to 'educate' drivers to check behind them before opening their car doors.
Nothing particularly new in my opinion.
"It is an offence to open, or cause or permit to be opened, a car door so as to injure or endanger anyone (section 105, The Roads Vehicles (Constructions and Use) Regulations 1986 and section 42, Road Traffic Act 1988)"
This is thus expressed as a ‘MUST’ in Rule 239 of the HC, which states that you “MUST ensure you do not hit anyone when you open your door”, with further advice to “check for cyclists or other traffic”.
This rule, however, is not well-known or publicised, and fails to adequately alert drivers to the specific risks to cyclists and motorcyclists if car doors are opened into the road without looking behind first.