I am the youngest of four kids and I have the lowest SSN of the group. I believe I got that number because my parents applied for the numbers all at the same time (the IRS was not as demanding back then when it came to claiming dependants on your return). Most likely some SSA clerk put my application on the top of the stack while processing them.
Conveniently, I am able to keep track of my brother’s SNNs since they are all 3 numbers apart.
As for databases of death certificates, states keep those as well, but they are not cross-referenced to any birth certificates for a variety of reasons.
You can look up deceased people’s SSNs through the Social Security Death Index on the web. Not every dead person with an SSN is listed however. You don’t get listed unless the SSA paid out a death benefit for you.