There's actually a rational reason to not worry about LEO space mirrors becoming potential space junk. They just have an extremely high surface area to weight ratio, meaning they're going to experience relatively high drag. If something like this is abandoned in low earth orbit, it would probably deorbit relatively quickly, like in a matter of months.

I'm not suggesting that this mirror is a good idea, it's unclear what the use case even is. But I am saying that it's not any more dangerous than any of the dozens of smallsats launched by University robotics programs. People don't tend to have any problem with that kind of satellite, though they're probably more likely to be abandoned relatively quickly and they would stay in orbit longer than a satellite that has what is essentially an enormous sail.