What is deadnaming?

Intentionally using the former name of a (typically trans) person with the intent to cause harm

That is incredibly niche. I’m sure 99.999% of the population would not know what that is, were it happening to them.

I mean, how many names of dead trans people would the general person on the street know?

It’s not specifically of dead trans people, they meant of all trans people. The term deadname typically refers to the birth name of a trans person that they no longer use.