So, it’s definitely a different scenario, but something similar happened with me and my mom. It was an email, not a letter, and originally from me (because I was a bit of a coward) but she responded with similar sentiments.

Part of my email (which was so long you could call it a letter) included how painful it is to be deadnamed. How it’s a constant reminder and a painful moment every time I hear it. She was receptive and responsive.

I recognize that I’m privileged to have an accepting mom that’s willing to work with me. That being said, I can’t help but recommend that you respond and point this out. It’s painful and hard, so no shame for not doing it if you can’t, but totally worth it if it pays off. All of our situations are different, so definitely give some thought to yours before you do this; but it solidly could be that she doesn’t realize what she did. It may hurt at first, but if she actually wants to be supportive, she’ll probably appreciate it.

Either way about it, I’m proud of you for coming out to her. It’s scary and hard. I’m glad she was (mostly) supportive. I hope you the best!! ^{.^}