Two of mine sheared off last time I changed discs. I never bothered digging out the threaded bit, just left them in. They’re just assembly aids, the centre wheel boss/bearing housing ensures the discs are centralised.
 
Like @Nodge68 , I use stainless replacements, but pozidrive ones I have in stock. For getting the screws off I recommend an impact driver, as in the one you smack with a hammer like this:

926881


Reason for that recommendation is that as you hammer the handle, you drive the bit into the screw profile ensuring it cannot slip out and round the screwhead, the impact also helps to shock the screw free to prevent it breaking as happened to @Rank Amatuer.
 
what you can do is drill the head of then with a pair of pliers turn the rest of the screw around until it come out, when re assembling put a little copper grease around the thread and the top of the screw, never leave them off as they hole it central of which I learnt about in the 1960s I left the screws out of a drum and several weeks later I wanted to know what the noise coming from my wheel was and the drum had worn round the wheel studs into an oval shape.
 

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