It's a bit overstated on FB groups. And also they are measuring it wrong.
It's basically part of the legal testing that car seats have to go through to be allowed to be sold - this is the diagram which I've taken from the ECE R129 documentation. The crash rig has to be set up this way. The child's face is completely irrelevant because this will vary depending on how bulky the seat is and the angle of the vehicle seat, the important point is part C on the diagram (ie, where the isofix points are or would be in a non-isofix seat) and then the back of the front seat, if you drew an invisible line vertically from both points and then measured this distance. In the crash test, cameras record the forward excursion of the crash test dummy's head and if it goes past this line, the seat fails the test.
It's worth noting in cheaper and more basic seats because those seats tend to just scrape under the excursion amount required. With brands that put more research and development into the mechanics of their seats (e.g. Britax, Cybex, Maxi-Cosi) you typically see much lower head excursion distances anyway.
Head excursion is considered important because in a crash the injuries are caused by what they call the "third collision" - the first collision is between the car and another object (another vehicle, a wall etc), the second collision is what physically stops the person's body from moving (seatbelt, airbag, car seat straps) but the third collision is the passenger's internal organs against the front of their skull, rib cage etc.
In order to reduce injury, you want to slow down and minimise the amount of movement for that third collision. And the way that child restraints try to do this is to slow down and minimise movement for the second collision. One of the ways to measure how well they do this is to measure head excursion.
Then there is also the other risk which the above link talks about which is potential impact on the front seat, dashboard or airbag, which is also a serious risk to car passengers in accidents, especially in the rear seats.
Under R44 the distance was 550mm (55cm) and under R129 the distance is now 500mm (50cm).
If you measure this in your own car, you will often find the seat itself is about 53cm deep. I've measured a few cars. So unless you have an absolutely teeny sports car where the front seats overhang the back seats, you're probably already achieving the gap without actually trying.
Also, it's arguably not a minimum distance to use the seat. Unless the car seat manual states a minimum distance, and some do say they prefer the front seat to be pushed as far forward as possible, it's a performance metric for use in crash test scenarios, not an operating instruction. You do of course need to install the seat to the instructions given in the user manual, but you don't have to conform to all of the exact metrics of the regulatory crash test - unless you drive at under 30mph all the time, you're not doing this anyway. It doesn't make sense.
It just seems to be an argument against forward facing. And while I agree that rear facing is safer than forward facing, this particular argument winds me up because I have never come across a rear facing seat which fits in a 55cm gap either. The most compact ones seem to take up about 70cm. And while this is also measured from a different point (closer to E on that diagram) I don't think that this constitutes 15cm of extra space. So to be perfectly honest I think this is just fearmongering.
If you have a small gap between front and back seats and you are using a forward facing seat, it's beneficial to have an R129 certified one and it's always important to follow the instructions for installation, get as tight and solid an install as you can and make sure the straps are positioned properly and tight enough. But it's not accurate to call it a "survival gap".