I personally wouldn't go as far with that claim, but young girls are certainly discouraged from pursuing STEM careers either actively or unintentionally (representation/role models, toys etc.)
What I (and I suspect others) want to see is proof of active discouragement, rather than the lack of encouragement.
The link between wealth, marriage suitability and social status is well observed for men, and in stereotypical pattern boys are pushed towards professions which maximize the potential for high income. Since society do not measure the value for women on how much money they bring, it follows that girls are not pushed with the same fever towards high paying jobs except if local situation causes families to do so by necessity (which is one explanation why certain countries have higher ratio of women in typical high paying profession).
I have the theory that if you want to get equal amount of young girls and young boys in STEM careers you need to remove focus on how such choice can lead towards high income. It would not increase the encouragement for girls, but fewer boys would be pushed in that direction and as a result the difference between the sexes would decrease.
That's a bold claim with zero sources. Consider this a "citation needed".