There's no guarantees of complete anonymity for sure, and I just had heard too many horror stories of women writing about sex getting stalked and women writing about feminism getting brigaded and doxed - so I decided to try to do what I could to mitigate that right from the start. I never talk about what part of the country I live in and all names from my personal life are pseudonyms - including my pets.
Medium has my real info because it needs a real bank account or debit card in order to pay me, and that could, of course, conceivably be hacked, but I feel reasonably safe. I have had a couple of men say they wanted to "try to find me" because they were attracted, but fortunately no threats (although a lot of other women I know on Medium who write about similar things have gotten them). Whatever the reasons for that, I'm grateful and I'm happy to not have to deal with them. But, I do appreciate that if I ever needed to just disappear from Medium without a trace, I'd lose all my followers, but it would otherwise be super easy to do.
In addition, Medium does a reasonably good job of warning and then evicting any truly problematic individuals and that makes me feel better too. I don't do all that much with Twitter or other platforms where they don't have good safety protocols. Be careful out there...