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?

Let’s say you knew someone named Bob, who later came out as a trans woman named Sue. If you insisted on calling her Bob instead of Sue, that would be deadnaming.