Dire Bare
Legend
So let's say you want to join a game. The DM gives you a list of accepted races that includes standard other than dragonborn and tiefling. They're up front about it, it's in the preview campaign documentation.
I ask because as a DM I want to have a campaign world that makes sense to me. I just don't see a place for dragonborn in my world. They've simply never existed in the history of the campaign world that I've run for a very long time across multiple editions.
Is that really a show stopper for you? If it otherwise sounds like an interesting campaign?
Hussar covered it pretty well. Without you going into more detail on WHY dragonborn don't have a place in your world, it sets a red flag for me. That you MIGHT be a DM who says "No" more often than "Yes", a DM who can be inflexible. Doesn't mean that's truly your style, or even if it is that we wouldn't have fun gaming together, but it's a red flag for me.
Now if you were able to communicate that you are going for a particular storytelling or genre feel, "old school" or classic literary fantasy where the protagonists are generally "normal" humans vs. a scary and monstrous magical world, with the occasional fey or monstrous ally . . . . I could get excited about that. Although even then, why not a dragonborn "Chewbacca" in an otherwise human party?
It's not that you don't allow dragonborn necessarily, it's how well you articulate WHY you don't. And the fact they haven't existed in your campaign yet is weaksauce. You mean you aren't creative enough to find a way to introduce them? In the Realms, dragonborn were introduced as a part of a planer catastrophe, they are refugees from another world! Or perhaps, as in Eberron, they are from a distant, mysterious continent that even the most traveled of explorers knows very little. Or, as in Dragonlance, they are a created race used to build a conquering army, but then discarded when their purpose has failed. In 3rd edition, dragonborn were humans (or humanoids) who served Bahamut and underwent a ritual than transformed them into platinum-scaled draconic beings.
Of course, your reasons are your reasons and as long as you and your players are having fun, you're certainly not doing it "wrong". And you get major points about being up front about it with new players (too often, that doesn't happen). But the way you've described your campaign's limitations in this thread, it would not get me excited to join your campaign, even if playing a dragonborn wasn't on my immediate bucket list.