The rarity of a species depends entirely on setting. Obviously Dragonborn are common in a Dragonborn-only setting.
Clearly. Setting can always change default assumptions.
If 5.0 explicitly called Dragonborn rare, that only applied to the 5.0 default Forgotten Realms setting. The 5.5 setting doesnt need to preserve that. Ghe Dragonborn can be common, especially in certain regions.
Looking at it, they are called out as uncommon, along with Gnomes, Half-Elves, Half-Orcs, and Tieflings.
"The dragon born and the rest of the races in this chapter are uncommon. They don't exist in every world of D&D, and even where they are found, they are less widespread than dwarves, elves, halflings, and humans.
In the cosmopolitan cities of the D&D multiverse, most people hardly look twice at members of even the most exotic races. But the small towns and villages that dot the countryside are different. The common folk aren't accustomed to seeing members of these races, and they react accordingly."
As you can see, their uncommon status is explicitly called as for every setting, but of course specific settings would change that. For instance my group once ended up going to the Council of Wyrms setting as part of their adventuring and Dragonborn were common there. In my Forgotten Realms setting they are extremely rare and are not a PC race, being far more powerful than the PC races. My group has only encountered one, and that one is the head of Candle Keep. They might have encountered the other one at the keep, but I can't remember.