This ended up really wordy, so have bolded the important points.
IME children between 135-150cm will fit well without a booster seat in some cars but not in others, whereas 150cm+ children generally get a good fit in all cars.
It also depends on whether the child is long legged, skinny, etc. But the seatbelt will be in slightly different places in different cars, the length of the back seat etc. The middle seat is often slightly smaller and slightly raised, which sometimes helps.
Also, even if belt fit isn't perfect, it's likely that the seatbelt will still give "good enough" protection over 135cm, which makes it a reasonable risk to take for occasional or short journeys even if you prefer to continue with a booster in your main car.
All of these are the reason that the UK rule is 135cm whereas some countries have it at 150cm. It means it's at your discretion.
I do think some of those "5 step" things are overstated - there is no need for their feet to reach the floor (though they may be more comfortable with a step to rest their feet on) and nobody sits perfectly still like a robot for an entire car journey.
The most important thing to look at is the lap belt - it needs to run over their pelvis, whether that means it lies on the pelvis itself or over the tops of the thighs touching the pelvis. If it's running over their tummy, then it's too high posing a higher risk of injury in a crash. A booster seat (backless or high back) "boosts" them up to correct this.
The second thing to check is whether they are sitting right at the back of the seat when they get this good fit. What you'll sometimes find is that DC obligingly sit right to the back to show you that the belt fits properly, but their legs aren't quite long enough to bend their knees at the edge of the seat, and it's not comfortable for them to stick straight out so they naturally slouch or scoot to the end to bend their legs. That moves their bum/pelvis forwards leaving a little gap behind them, and so the lap belt moves up onto the tummy rather than the hips, which is not a good fit. So these DC would benefit from a booster seat for a bit longer, even though they might be absolutely fine in a different car with shallower seats.
If the belt is too high on their neck so that it's rubbing, some DC will find this annoying and want to pop their shoulder out of the belt or place it under their arm. This is very dangerous in a crash because it means the belt won't pin them back effectively, meaning they could hit their head on the front seat or be hit by the inflating airbag (sounds harmless, but it's not). They can also get internal injuries from the belt itself, if it's under the arm. So again if they are doing this they still need some kind of booster seat. Most of the aftermarket solutions sold to solve this issue are not safe - they try to pull the shoulder belt down by attaching it to the lap belt, which pulls the lap belt up. If your child still physically fits in/on a booster seat, a booster seat is a much preferable option compared to any other product on the market.
If you are an adult who is shorter than 150cm or finds seatbelts too high/uncomfortable and you can't adjust it any lower in the car, the "Klunk Clip" or Carpoint Belt Clip product is a safe solution though it is NOT preferable to a booster seat for children.
Children can be too tall for a high backed booster - if the booster won't adjust any higher and their shoulders are higher than the belt guide, then it's too small for them and you should stop using it. Also they can be blocked from adjusting higher by the sides of the car roof being too low. And as several people have mentioned, some HBB seats aren't really designed for comfort at the larger stages. If you want to keep your child in a high backed booster it's likely possible to buy a taller one and/or with a deeper leg rest - just go and measure them in a shop/take your child with you. Anecdotally, Joie do one which adjusts (Traver) and Britax are fairly deep in the seat, the higher end Cybex ones adjust wider for shoulders, and Diono are good for wider hips. Cheaper brands and combination type seats tend to also be quite short, also Maxi Cosi and some cheaper Cybex.
But a backless booster is a good option for older children if you don't want to buy an expensive high back, to alleviate embarrassment, or a low cost/more portable option. It's recommended that children should be at least 22kg/125cm in order to change to a backless booster, but a good quality high back does offer better protection if it fits safely and is properly adjusted.
Children over 36kg can still use booster seats - this weight limit is part of the old R44 standard and should be ignored if there's no harness in the seat. (Question mark over combination seats - a lot of manufacturers do prefer use of these are discontinued at weights over 36kg). If preferred, you can purchase a R129 standard booster as these are allowed to have higher weight limits or specify no weight limit, only a height limit.