Well I guess nobody really knows for sure how many small, tasteful "gender reveal parties" are happening behind closed doors. It is theoretically possible that this happens all the time.
However, the whole idea of having a party for the express purpose of telling your family and friends which type of genitals your unborn baby has (I mean, let's face it, there are only really two possibilities here) is one that seems rather as though it was invented to be shared on social media. Especially if you're going to go to the trouble of doing something like cutting a cake to see whether it's pink or blue inside, or some other kind of gimmick for the big reveal moment. It would almost seem wasteful not to put it on Facebook so that your old classmates you haven't seen for 15 years and your old work colleagues and that girl you met at your friend's hen do can all see it and give you that validating thumbs up and a "congrats hunny!" on Facebook. It seems like the whole point of a gender reveal party is to draw attention to yourself and your pregnancy, so why wouldn't you put it on social media?
So colour me sceptical about there being a lot of low key ones which we don't know about because they're not being splashed all over social media.