I understand that historically. However, I would argue that those restrictions were actually by sex as changing gender was not really an option back then. Even if one tried, I assume most cultures would still enforce those ideas based on sex.

We have very few restrictions on gender today, and the ones we do have still tend to exclude transgender individuals (combat roles, some levels of some sports, some bathrooms). So I'm having trouble understanding in general what a tangible benefit would be. For example, what changes for a man that dresses to look like a woman if they say they are a man vs they say they are a woman (or some other identity)? Are all the physical attributes and actions the same? If someone wants to be a specific gender that's fine, but does that actually change anything they could or couldn't do as their prior gender in today's world?