Deadnaming is when you refer to a trans person by the name they went by before transitioning.
It can be incredibly painful for a trans person to remember the part of their life when they couldn't outwardly express themselves as they felt on the inside, it can feel as if any progress you've made since that previous part of your life has been suddenly invalidated simply by hearing or reading the name. It can hurt to even know that people know your deadname, even if you know they'd never use it. Knowing people even have that mental connection when it comes to your deadname can be a horrible feeling.
It may not directly put the community at risk, but it's incredibly hurtful to many members of the community who either know firsthand how it feels to experience that, or to members who have trans friends or family members who've seen the toll it can take. Thank you for asking, and if you have any more questions please feel free to ask.