It's been around and discussed for quite some time. I'm still searching for a definitive answer on which is right, true or magnetic on the MiG-21bis, specifically.
Viper, a sim enthusiast that I have a lot of respect for through these forums, had this quip that leads me to believe it's true. However, I do not know his references:
Full thread here: https://forums.eagle.ru/showthread.php?t=131025
Conversely, the following answer came from some educated pilots indicating magnetic, but I cannot guarantee their knowledge of this specific Russian aircraft.
https://www.quora.com/What-do-all-the-controls-in-a-fighter-jets-cockpit-do