It is pretty common for private planes to be removed from Flight Radar and other websites - the FAA allows owners to request this and publishes a list for companies to consume.. it was likely gone from there for a long time. But the data is still broadcast as the plane flies and some sources don't filter them out such as ADSB Exchange (https://globe.adsbexchange.com/)

Ref: https://www.vice.com/en/article/n7zygg/twitter-bans-elon-mus...