I think there is a bit of a blind spot currently among older primary aged DC who are still well under 135cm but aren't using booster seats. My theory is that it's because this rule change (to require the use of booster seats up to age 12/135cm) happened nearly 20 years ago, it's completely old news and has stopped being reported on in the media, advertised prominently in car seat shops etc. The assumption being that if the law changed 20 years ago everyone should be aware of it by now.
I think this is a false assumption, because the problem is that parents of DC aged roughly 6-10 are largely over 30, meaning in 2006 the majority of them were over 12 but likely didn't have DC yet so this law would not have affected or interested them at the time and they most likely weren't even aware anything ever changed. If you were born in the 80s or 90s then car seats were a totally established thing for babies and very young children, but booster seats were barely used at all and so I think while the vast majority of parents in this age group take it for granted that younger children must use a car seat, they don't necessarily realise that there is an upper age limit that is as high as 12 or an upper height limit at all, and they stop using them when they think a seatbelt doesn't look massively uncomfortable any more, which tends to be about 120-125cm or age 7/8 ish based on my observation. Younger children look obviously too small to sit with nothing and have the seatbelt on their neck, but most people are not aware of the fact the lap belt position is the more important part and can cause injuries when it sits too high on the soft part of the abdomen, which it does for most children until at least 135cm (often taller).
If you are aware of the law because you looked it up, or because you were aware of the change in 2006 because of younger siblings or older DC, you are in a minority and I don't think the lack of booster seat use is a lack of care, I think it is a genuine lack of awareness that it does continue to make a difference up to certainly 135cm but potentially 150cm ish depending on the specific car/child/seatbelt combo. I also think this will start to self-correct coming into the later half of this decade as parents will be aware of the rule about being 12 because it was very funny to taller 10/11 year olds at the time and absolutely mortifying for shorter ones, so anyone born after 1995 is likely to remember it.
135cm is pretty much bang on the 50th centile for girls aged 9.5, so statistically, about half of them should still need one whereas the other half won't, but if all parents are not using them, it's likely most have stopped too soon. 128cm is on the shorter side for 9.5, but not massively so. I also think backless boosters have a really practical role at times and while ideally the parents of the other DC should provide one it is less annoying to carry the smaller kind around. My personal philosophy is about weighing practicality with risk management so I'm happy to step down a level of protection for a shorter or low risk journey whereas I'd have a more robust set up if possible in our own car. For example we use a car sharing scheme and today I let DS2 age 7.5 travel with no booster so that DS3 age 4.5 could use the high back booster (of dubious age and condition) which comes with the car so that we could drive literally 3-4 mins around the corner at 30km/h to avoid a 15 minute walk in the freezing cold. I was totally fine with this as a risk, even though DS2 was actively quite worried as it was his first time ever without a HBB. If we were doing a day out or longer drive, I tend to bring the harnessed car seat for DS3 because he leans out of the seatbelt otherwise, and DS2 either uses his own HBB or the included one if it's in decent condition - some of them are brand new.
Equally, when DS1 was 8ish and not all of his friends' cars had spare boosters, I would weigh up the length of the trip and the ease of providing a booster, and if it was simple enough then I'd try to give one, if they had one I'd be grateful and if it didn't work out and it was a shorter trip I didn't worry too much about it. It's obviously important that they wear a seatbelt. A booster seat is ideal, and he always used one in our own car, but by age 9 the amount of situations where it's life or death is diminishing.
For some context - these figures come from the US where it's not a legal requirement in most states, and are not perfect for various reasons, but when you compare children age 8-12 in normal observed use vs children of the same age category who died in car accidents, the normal use of boosters of any kind is around 13% compared with 5% of the children who died. This shows that it does make some difference.
However the figures for no seatbelt at all are much more stark. For the same age group, the normal observed amount of children who travel unrestrained is about 16%, whereas 50% of children who died in a car accident were not wearing any kind of seatbelt. So wearing a seatbelt is the important part. Using a booster seat is absolutely worth it but is secondary.