Anti-hail netting is definitely a thing (protecting cars or fruit trees), for the reasons you state. Even big hailstones are rather slower than fast golf balls.

Hail damage at car dealerships in my area is so expensive that the dealers have added coverings for the lots (pretty sure highly encouraged by the insurance companies), and they are most definitely not nets. Nets would have been cheaper, so that should say something about their effectiveness (or not)

Ascetically, nets get dirty and are hard to clean, have birds on them, things grow on them, and they look gaudy to some.

Hail protection nets are a COTS product. Full stop. Properly-designed nets are exceptionally strong; that's why we make "bulletproof" vests out of them (that's all they are, just a fiber net with a very tight weave).

Just because your local car dealership(s) didn't opt for hail protection nets isn't on its own evidence for or against their effectiveness. There are a great many factors that go into such a decision and, unless you were privy to the decisionmaking process itself, whether or not nets were considered -- much less their effectiveness -- is pure speculation on your part.

I agree with your implicit point that the key question is probably cost effectiveness compared with insurance. However, in the context of the OP, insurance premiums in hail-prone regions was listed as a prime reason why single-axis-tracking panel installations are, in some regions, probably still a better bet at grid scale.

The point I'm trying to make is just that there are a lot of off-the-shelf (or nearly so) hail protection strategies that seem like they might have better economics than single-axis-tracking installations, which might improve the cost effectiveness of a fixed panel installation to the point where, even in hail-prone regions, it might be doable.

To be clear: anti-hail nets are already a COTS product, just not for this application (as far as I'm aware). Nets are just really, really good at absorbing kinetic energy. I mean, that's effectively what kevlar ("bulletproof") vests are made of -- netting with a very small cell size. But it seems to me like some kind of roll-out kevlar or dyneema netting could be really effective at protecting panels, though I think you'd need some kind of better strategy for safely dispersing the hailstones after they were caught, so you don't end up with a huge collection of hailstones on the net.

I agree that there's probably too much light blocking to leave them up continuously, and I'd also be worried about UV damage. But remember that the comparison here is to single-axis-tracking installations; I think automatic deployment of anti-hail netting could easily be made at least as reliable as the tracking system, if not significantly more so. Maintenance and testing would also be very cheap in comparison, since you could do it at night (ie without affecting power production), and damage to the protection system could be repaired without taking panels offline.

The more I think about it, the more I'd be interested in seeing it deployed.