I've found this term in poor taste ever since I saw it, and this article I've seen on Twitter is a great look at why.
rachelemoss.com/2021/06/24/a-letter-to-trans-widows-from-an-actual-widow/
Marriages end all the time because one spouse isn't who the other thought they were. It's sad or heartbreaking or difficult, and people have (obviously) got the right to grieve for a relationship which is no longer the same. But it isn't a death and it's thoughtless to describe it as though it is.