A sleep sack is safe for newborns and babies as long as they're used properly.
However if the room is too warm then the sleep sack can cause the baby to overheat but using a sleep sack is much safer than blankets which a baby should not have until over a year of age.
Sleep sacks when used properly can help babies sleep safely by decreasing the chance of suffocation like blankets would.
Footed pajamas are also another good option for keeping a newborn child under under 2 years of age warm.
But even then footed pajamas are good for toddlers of all ages and helps keep them warm but I would not recommend the footed pajamas once the toddler is using the potty and out of diapers because it can be hard for the toddler to remove the pajamas to go potty on their own.
But if the child is diaper aged then they won't need to remove the footed pajamas since they'll be using the diaper instead.