Actually this is a rather 'thin' pop version of an idea proposed by Peter Carruthers. His 2011 book "Opacity of Mind" definitely is not a light read but makes a very compelling case indeed. If this is your jam, I highly recommend this book.
I run this site : https://ducis.net/
It is a hobby that got out of hand. The idea was/is to see what this data reveals. What can be learned just by observing 'traffic'.
The answer is : a lot and I know various agencies and governments freaked out when they realized how visible some things are. Basically, with enough 'training' and domain-knowledge it's all pretty much out there : military transports, private military contractors, refugee flights, mercenaries, embargo busters, the spooks etc.
Obviously there was a reaction. Over the years they've tried to tidy things up with some success by putting pressure on the tracker-sites (or simply attacking the small players' servers).
Anyway, the point is : it is complex and fascinating.
I'm not interested in shouting "SECRET flight..." for clicks but if someone wants to try and set up a new 'ADS-B Exchange' : basically free, responsible, open source etc and wants to take on board a couple of years of deep domain-knowledge and decades of coding skills, contact details are on the site.
I know this is like a shameless (well) plug but...it's complicated ;)