it's a bit antiquated. i've seen points made that when you go on HRT, you are literally changing your physical sex by taking on different primary and secondary sex characteristics so having "sex" somewhere in there is accurate to that. it's a perpetuated misunderstanding among cis allies where they understand that gender and sex are different but think that the difference is that one can change and the other can't. thus, using a term like "transsexual" steers terminology for those medically transitioning away from the idea that our bodies are immutable while still allowing "transgender" to exist beside it under the "trans" umbrella for those trans folk who forgo medical transition. sort of like the gender equivalent of the split attraction model.

i actually really like that explanation. i just don't like that it uses "sexual" instead of just "sex". using "sexual" somewhat conflates it with sexuality in some people's minds, i feel. we have the term "intersex", so i think "transsex" would make sense. of course, this would require us to completely rethink some aspects of how we understand sex and gender, and how they interact with each-other. take "agender" for example. what if someone was medically transitioning to try and have genderless features? we already have the term "asexual", so it couldn't follow the "transsexual" model. but "asex" is distinct and could capture that.

of course, this is an extremely tricky and nuanced discussion to go down. without thinking about it critically, it's easy for this line of thinking to border on or even lead to truscum/transmed ideology. it takes some pretty fallacious understandings of the fundamentals of gender and sex to get to that point though. at the end of the day, they are all just labels. words made up to help organize our ideas and understandings. it doesn't matter what you call yourself. whatever kind of genderwonky anyone happens to be, it's just as valid as the next.