I once flew a Citation X across northern germany, was asked by Maastricht if we would agree to be intercepted for training purposes.
Sure we were. Flying at FL470 with Mach 0.92 at that time.
Like 15, 20 minutes later we noticed no interception still, and inquired with ATC.
Reply: they couldn't get you, too high, too fast, you were already way past them and they couldn't go past Mach .95 without risking a supersonic boom from some parts of their planes...