Since the start of the month, Flightradar24 has been pulled from app stores in China as part of the recent renewal of the anti-espionage laws unveiled in China 7 years ago, providing a basis for safeguarding its national security. Whilst those who had the app before being unlisted are not affected, updates to the app are no longer pushed.
This has been done supposedly on the grounds that the equipment FR24 provides to users, namely ADS-B receivers and whatnot, are transmitting vital aviation data within its airspace, especially that of its military fleet, thus threatening the security of its notoriously secret operations within the PLAAF and having the government consider this as means of espionage and spying. The said equipment has been pointed out by local security organisations, launching individual investigations and seizing hundreds of the equipment that broke regulations.
According to authorities, the amount of receivers have the ability to provide coverage for military aircraft performing crucial operations in restricted airspaces to anyone at any time that is willing to look at it, including international organisations that might want to get their hands on the information, which has been judged to be considered a direct threat that would lead to catastrophic consequences.
It is also noted by local bloggers that Flightradar24 is currently working to put the app back under regulation, as well as hoping to have it relisted back to the Chinese public.
Sources:
https://user.guancha.cn/wap/content?id=613976
Also cheers @Alexander_Nikitin for bringing this topic up, much appreciated.