Booster seats with no back support: can they still be used?

Is my understanding correct that:

1)new (produced after 2017) booster seats for children < 125cm must have a back support, BUT

2)older booster seats which do not can still be used? I.e. the ones that are a sort of cushion that simply raises the child, like this one www.trunki.co.uk/collections/boostapak

We’ll be flying with 2 checked suitcases, an infant, a 4-year old (<125cm) plus pram and car seat for the infant. Being able to carry a small backless booster seat for the 4-year old would be great, because I honestly don’t know how we could carry her gigantic car seat, which must weigh some 9 kgs!
We could buy a cheaper and lighter one (e.g. Graco makes some, they’re on sale at Amazon and Halfords) but it would still be very bulky. The logistics of carrying all this paraphernalia at the airports would be quite a nightmare: one adult pushing the pram, the other adult pushing a trolley with suitcases + the infant’s car seat, the 4-year old carrying her Trunki…

Of course I get it that a bigger bulkier car seat with back support is safer, but it’s a balance…

Plus, we won’t be renting cars, but will rely on minicabs to and from the airports (both here and abroad), so we can’t rent a car seat from Avis/Europcar etc.

We’ll be flying in the EU, where the laws on car seats are AFAIK the same.