“can i still call you ___”
“but you make such a pretty/handsome girl/boy!!”
“you’ll always be [birth name] to me”
“i’ll still call you [birth name], it’ll be easier”
Advice: Don’t surround yourself with people who do this, even if they were your best friend they were never worth it. And they clearly don’t see you (as a best friend) the same way as you see them if that’s how they treat you.