I think the poster in the middle is talking about rear facing seats for babies, and quoting a Google AI summary which has garbled the advice or possibly mixed up UK and US guidelines, but you and TeenToTwenties are talking about not having seen RF seats for older children.
Those were not widely available in the UK 20+ years ago. All you could get were seats which went RF up to 10kg or 13kg depending on whether it was before or after the Group 0+ category came in, and then they could be turned FF from 9kg and used up to 18kg. Those lasted a bit longer than an infant carrier by height but that was it really.
The In Car Safety Centre was importing some seats from the Nordic market in the late 90s/early 00s (funnily enough Britax was making those seats here in their UK factory, but only selling them in Sweden, Denmark and Finland!) but they were a specialised retailer and the seats were sold for children with special needs who needed more postural support, rather than as a safety benefit for all children. If other places were importing them, I don't know. I was not around in those circles on the internet then. You can look at old MN archives though if you're ever interested (it's fascinating to put in any keyword you like TBF)
It was mainly thanks to American parenting/car seat forums online, where a debate was raging over whether it was safe to FF at age 1 or wait for age 2, and awareness raising by the families of some children who were unfortunately injured or killed as a result of FF seats used too young that the idea of RF for longer started to become known, at first in small corners of the internet and then spreading wider as the internet became more widely used really. There were also a couple of online bloggers or activists who had moved from a Nordic country to another one (often UK/US) and were then confused and dismayed by the lack of availability of RF seats so decided to campaign for them.
By the end of the 2000s the In Car Safety Centre was not the only ERF importer in the UK, they had moved their ERF seats out of the special needs area to a new ERF area of their website, although you still had to drive to them for a fitting. It was also possible to buy seats directly from Sweden online. But that was basically the only way to get a rear facing seat for a child over 13kg, you had to have a specialist nearby or be willing to order an expensive and possibly complicated seat directly from abroad, remember online shopping was not at all the norm at this point, and online video was only just starting to be used for things like car seat installation demos. A lot of RF seats for the Nordic market use lower tether anchors, which are normal there but not well known at all in the UK even now.
But certainly through to about 2014 it was virtually unknown by the vast majority of people. Then some of the more mainstream brands (Cybex and Britax) released rear facing spin seats and things took off then. Joie made them affordable a few years later and now they are totally normal, and more people RF to age 2 or 3 which was virtually unheard of 15 years ago. TBH, 15 years ago most people thought it was extremely strange to keep babies rear facing past their first birthday. Now it is the law, even if the law only says you must RF for 3 months past that date. It seems it's enough to get people to switch to a second stage seat which at least has the ability to RF and therefore it is a no brainer to keep doing it if everyone is happy. Which I think is great. I'd much rather see a lot of people RF a little bit longer, than a few people RF for a lot longer.