There are many pieces now that proclaim biological sex (not gender) is a social construct. There are pieces in Forbes, Psychology Today, Professor curriculum, etc that are now saying this. I would say it was fringe if it weren't for the fact that it's been elevated by major publications.

When I was younger "gender is a social construct" mostly meant that if you were a man you were allowed to enjoy feminine things and if you were a woman, you could be a tomboy, etc. It also applied to trans individuals. However, that has shifted heavily over the last few years to be something very different and has gone so far as to go fully to the extent that sex (not gender) at birth is up for debate (even outside of Turner Syndrome or Intersex births.)

Orthogonal means uncorrelated. Sex and gender are in reality highly, though not perfectly, correlated.

Concur. What’s a better word/phrase?

I’d say “not coextensive.” Unfortunately no single word that means exactly that.

I would say 'orthogonal' is used for conceptually independent, not causally independent nor statistically independent (uncorrelated). So i would say that sex and gender are conceptually independent, although in reality statistically highly correlated.

I think the word you're looking for is "distinct."

The earliest most popular publication that posited that "sex is a social construct" might be Butler's Gender Trouble. This article is a pretty good overview on the evolution of the discourse on this topic:

https://plato.stanford.edu/entries/feminism-gender/#SexGenDi...

(I linked straight to the section on sex as a social construct, but the whole article is worth a read.)