Yep it's gone for phone anyway. I upgraded my 640XL to win 10 at the weekend, and restored it back to 8.1 after a day only to discover that Flightradar refused to re-install and that it no longer exists in the store. When I contacted FR24 support they had this to say:
"I'm sorry to inform you that due to very small portion of Windows Phone users, Flightradar24 was forced to make a difficult decision to stop providing the app through the Windows Phone App store. If you've previously purchased the app you may be able to download and install it still by going to the Store --> My Library --> Apps --> Show all and selecting Flightradar24.
If you have more questions, please contact directly with the Windows App store.
Kind regards,
Flightradar24.com Support"
The suggestion did not work, and additionally the Microsoft chat bot clown was completely useless repeating the company line that there was nothing they can do.
This rubbish didn't happen in WM6.5 when you could do a proper bloody backup and restore of your own damn device.