Feelings I had for my classmate were obviously one-sided (pretty sure of it), so I made a reasonable decision to never confess to not make things awkward. In a few years since we graduated I finally moved on, although I never loved anyone this much since.
Yet all this time I had a desire to tell her about them. I would definitely want to know if someone loved me this much ever. Would she? I don’t know. I don’t even know if she is in a relationship right now or not.
I would really want to cite some scientific study that “Over 80% of girls have their self-confidence lifted after being told they were secretly admired (p<0.05)”, but can’t find one.
I feel like if you are still friends, saying “ooh I had such a crush on you when we were younger” is ok, I’ve been on the receiving end of that and it was flattering and surprising because I absolutely did not feel attractive when young. And it was not threatening because he was in a happy relationship. Since you are so out of touch with her you don’t even know if she’s in a relationship, no, I would find that weird.
We send each other birthday congratulations, mine are often rather lengthy. I was planning to attach this information to one of them.