Modern? It's over 800 years old.

Moding, be it boy or girl, is presenting as that gender, regardless of internal gender.

As an example, a trans woman may sometimes choose to present masculine for whatever reason. That would be boymode.

However, a trans man choosing to present masculine is also boymode.

Obviously, it would follow that non binary people or gender fluid people would also be able to choose to present masculine and thus go boymode.

The then exists as an expression of presentation, regardless of birth assigned gender, or genital/gonad configuration at the time of moding.

I'm cishet male, and could girlmode if I so choose, as the term does exist within that frame of reference as well, albeit rarely. It isn't even about passing; my giant bearded self could still girlmode, and it would be an accurate usage of the term still.

That's actually part of what the greentext implies; the Lord of that realm isn't passing, but is still boymoding, and his subjects respect that once they're made aware of it.

That all being said, it is rarer to see the term used when presenting as your affirmed gender. It would typically only be used when presenting as your assigned gender. It's also unusual to see it happen once someone is well into transition as there's less and less point in moding assigned gender.

The caveat in that is that it is a slang term. It may not be used exactly the same everywhere, or even reliably so in the same locale. I've known people that reject it applying to anyone that's cis, as it isn't the same thing for a cis person to present as a different gender than their assigned one. There's a totally separate set of social mores around it, and that does matter. It's also fairly rare for cis folks to present as other than their cis gender, and it wouldn't be for the same reasons. But I have heard and seen it used in that context, so I included it here to be thorough.

I've always seen the former but I suppose it could be either