I would advise against sending letters to exes, too. (Does anyone even send letters anymore?) I’m a fan of writing it out and keeping it to yourself or putting it on your blog, but big verbal gestures are best done face-to-face or not at all.
That said, I think sometimes people need to do what they need to do, so if that’s the case, just write a short message (five sentences tops) and send it in an email, on Facebook (if you haven’t been blocked), or on Instagram (if you’re not on it, create a temp profile and slide into his/her DMs). Then forget about it. Don’t expect a response, and go on with your life.
I recently took the Instagram approach. I only reached out to say hello. I didn’t make any apologies or even refer to our relationship. He didn’t respond and perhaps never will, but I’m not broken up about it because although I wouldn’t say no to a reunion, I did it more for my own peace of mind than to rekindle anything.
When it comes to past-tense love that’s still kind of present tense, I always say go casual or go home. If your feelings are too intense to go casual, you’re probably not ready to reach out.