It's very much a dated term from a bygone era of our understanding of gender. I don't necessarily begrudge anyone for using it, but in my opinion as a term it doesn't really make sense. Sex is also a socially constructed concept, and is not rooted in any one particular feature of our bodies or genetics. You can be of one sex and have the anatomy we commonly attribute with another, and legally speaking sex is divorced from genitalia.

Your sex can also be measured in many different ways, even from a purely biomedical stand point. You have phenotypes, blood hormonal sex (endocrine sex), secondary sex characteristics, and so on so forth. And when it comes to social sex, how we socially define sex, it all comes down to perception and the self-reported sex of the individual. I identify as a trans woman, as trans female, and both a woman and female independent of the qualifier "trans". Those terms describe how I see myself, and how I have seen myself since I accepted that I was transgender.

Equating sex 1:1 with genitalia doesn't make any sense. Where I live its possible to have your sex designation changed before having reassignment surgery. This is how it should be everywhere. And ultimately the only time where your assigned gender at birth should be relevant should be within the private boundaries of a medical office. And even then, doctors should be aware of the way that transitioned individuals have unique health requirements that are totally divorced from cisgender people who share their assigned gender.

As the moderator of the board I think anyone can use whatever terminology in self reference that they are comfortable with, and I don't think any particular terms should be forced onto someone. However any usage of the term "transsexual" to mean post-op in reference to anyone but yourself is against the rules as a form of gatekeeping and will not be tolerated. If someone who is post-op prefers that term for themselves that's fine, but going around declaring that someone is or isn't a transsexual because they have or haven't had bottom surgery isn't okay here.

Edit: changed wording at the end to be clear that choosing to use the term yourself is absolutely fine.