The corner speed is around 330knots if you want to turn better than the enemy is possible to lower the flaps?
First, the 'legal' reason: the flight manual doesn't offer this method. So you are not supposed to do it.
Second, there is a technical reason: MiG-21 had so called 'floating flaps' (as they call it), which are held down only by hydraulic pressure. The pressure is set such that the air dynamic pressure would retract the flaps as the airspeed rises above certain point even if the pilot forgot to do it, preventing their damage. I can't find the official threshold, but from memory the flaps would fully (or almost fully) retract at 600 km/h. This is a bit more than 330 kt, but anyway this leaves only marginal available extension at the optimum speed. (Full extension limit was around 340 km/h, but we are not talking about full extension in any case).
Now, what if we did it anyway? Floating flaps make calculations uncertain, but we only need to know if flaps would improve things. For that, we need to know if the turn1 is limited purely by the available lift or by something else. This 'someting else' could be thrust, structural load limits or available elevator deflection.
For MiG-21 (and most fighters), elevator becomes limiting only at supersonic speeds, and structural limits at high subsonic speeds (above the optimum turn speeds).
Thrust is important: we can be pretty sure that flaps will increase drag more than they will maximum lift.
Now, I don't want to post a graph for every statement I make, but all the data is taken from the official flight manual for MiG-21bis (the latest major modification of the fighter). I believe it can be easily found in English, at least for the export versions.
At low altitudes, where the turn radius is actually minimal, the data shows that the available load factor significantly increases with thrust (e.g. 2.5G at max military power vs 3.5G full afterburner at M=0.5,2 which is approx. 330 kt), and the turn radius decreases (~1200 m vs 800 m in the same conditions). This confirms that it is thrust that actually limits (the steady state) performance. Thus, flaps will not help. This is a rather typical situation.
However, turn radius keeps decreasing at lower speeds. (At low altitudes. At higher altitudes, it has a minimum at M=0.5 or more). The official data starts at M=0.5, but if I extrapolate a bit, the speed at which it stops being thrust-limited seems to be quite low. There is a chance that we'll get an optimum somewhere between M 0.3 and 0.4, but it will be a very low-energy turn.
Arguably, if we want to out-turn an enemy, we need the maximum turn rate (= minimum turn time), rather than the minimum radius. This optimum is at much higher speeds (around M=0.8-0.9)3, and there it is also mostly thrust-limited. There is no talk of flaps at such speeds anyway.
1 Of course, we are talking about steady turn. If you just want to turn around the quickest way, you are better off doing a chandelle, an unsteady maoeuvre.
2 At H=1 km (3000 ft), weight 7500 kg (fairly low but reasonable), with two missiles.
3 But this optimum is quite shallow, with little difference in the range M=0.6-0.9 at low altitudes. At higher altitudes, the optimum shifts clearly towards M=0.9. But all this only with full afterburner.