The issue of car seatbelts fitting children in the "grey area" height wise (over 135/140cm) varies hugely by car.
My old 7 seater was "van style" and the lowest setting on the seatbelts was only just low enough to sit in the right place on me (168cm)... I automatically adjusted the belt right down on my new car and it was way, way too low, and I can have it in the middle setting to sit correctly.
My 145 ish cm older children still sit on booster cushions (Germany) but the seat belts might just about be in the right place on the ordinary seat with the belt in the very lowest position actually - it would just be illegal here.
I don't think the belts could sit right on a 135cm child without a booster of some kind though - 135cm seems very teeny tiny indeed to me (as the mother of a tall and always broad shouldered 144cm nearly 9 year old who still uses a booster and a 120cm 5 year old :o )
The combination of the 36kg weight limit on most seats (including ones sold here in Germany) and the 150cm/ 12 year cut off age is weird though I think as according to the chart in the back of my DD's ancient "Red Book" at least 25% of girls will be over 36kg at 10 (my nearly 11 year old DD is 35kg). I suppose the assumption is that a 36kg child will also be over 150cm, but the 75th centile line on the height chart indicates a 144cm tall child, and the same line on the weight chart indicates 36kg, so statistically 1/4 of children must be over the weight limit for both hbb and cushions whilst being under the height limit for going seatless in countries with the 150cm guidelines.
I also wonder about the safety implications of a child being over the weight limit for their seat, although "Which" says to by height where they are under the height limit but over the weight limit... There must be loads and loads of kids sitting on hbb and booster seats they are too heavy for...
Obviously children are different shapes and some are too tall for a hbb at 140cm - it depends how much of their height is leg and how much body, and how broad their shoulders are - it is obviously a matter of common sense - when the child has to curl their spine or sit slightly sideways on the highest setting for a hbb they are too big!
The only hbbs for children over 36kg seem to be American, like this one which goes to 45kg American link - Amazon UK for 4 times the price but appears to have been discontinued by the manufacturer but is still on sale.
Otherwise UK and European seats mostly/ all seam to have the 36kg limit...
I do wonder why the 36kg limit is written on the seat if it has no acctual meaning, given that a 149cm 11 year old who weighs 44kg would be well within the healthy weight range...