Glen, looks like you have been reading the website of the British AA: https://www.theaa.com/european-breakdown-cover/driving-in-europe/country-by-country
Any rented car will have all the legally required items in it. The AA site is designed for Brits taking there own car "abroad". So it lists the extra things you need to buy for your car which are not required at home in the UK.
Headlamp beam deflectors - this is to do with dipped headlamps. When you dip your headlights, it doesn't just lower them, it diverts them towards the edge of the road to help you see the kerb and keep the light away from oncoming drivers on the other side of the road.
In the UK, where they drive on the left, headlamps dip to the left. In mainland Europe they drive on the right and headlamps dip to the right. If you drive a UK car in mainland Europe, your headlights will be dipping in the wrong direction, straight into the eyes of the oncoming driver. You fix this by attaching a small sticker or filter to the front of the headlight which cuts off the part of the beam going the wrong way.
Totally irrelevant for rented cars, which will have the correct headlamps.