Hi, I use the Raspberry Pi client supplied by Flightradar24 to feed them ADSB data. There was a thread on their forum a while back complaining of excessive DNS queries. It looks like that was a one off mis-configuration, but it prompted me to look at the NTP queries the client makes. As far as I can tell it has six 1..ntp.org addresses hard coded and does eleven NTP requests to those servers roughly every ten minutes, which I think sounds a bit excessive.
I’ve been trying to persuade their tech support that a) they should apply for a vendor zone and b) they should reduce the number of queries they make to the level given in the pool guidance pages, but without much success!
I’d encourage you to contact FR24 support and ask them to review their NTP Pool usage if you agree their use is excessive. I’d guess there are in the order of low tens of thousands of clients running Flightradar24 feeding software on various platforms - Pi/Linux/Windows etc.