Flat laying car seat

Please has anyone used a flat laying car seat for new borns before? I heard it safer cause baby lyes flat and it helps so the baby doesn’t slouch districting oxygen flow. And best for their back.

Any ideas please? Have you used or heard about flat ones before? Thinking of getting one for 0-6months. Couldn’t put my daughter on the one that tilts until around 7months and then she refused to seat on any for months after. To a long time to get her used to it.