Sleep bag in car seat

They are still not recommended. Just pop a blanket over baby once they’re strapped in 😊

These blankets are safety tested if you're open to other options: https://morrck.com/?gclid=CjwKCAiA-vOsBhAAEiwAIWR0TTbHHRNytohtQA2P0jt6j7Gg-9ymz3z_EM_feiaBkgH-3gldGbkUKxoCjTUQAvD_BwE

Anything that has to route through the car seat harness is not car seat safe. Thin layers under the harness, bulky layers over a tight harness.

I would never think of putting a sleepbag on for in the car. The car seats can cause them to get quite warm around the back area. Blankets are best once little one is strapped in, as so much easier

I used to use a thin knitted all in one or you could use a thin fleece all in one. Has to be thin with no hood. Then a blanket over the top if needed

They've only been crash tested on 4 different seats, so really they can only safely be used in those 4 seats.

but that's four more than required and it passed all of them.
There's no such thing as 100% safe with babies and children, so it's about weighing up the risks. These would likely be safer than a padded suit.

Yes but it's only safe and approved to use in those 4 seats. You shouldn't add anything to a seat that hasn't been approved by a manufacturer, these 4 seats are approved to have them in and no others.
Indoor clothes and blankets over the top of the harness are a lot safer again