Definitely not! I downloaded the itch.io desktop client, rolled back to version 0.9.9 through it, and everything went back to working exactly as before. The game is stable again, and I can turn the shadows back up to High. Both the GPU utilization and the dropped frames have returned to their previous, better metrics.
So, it really does look like a performance regression. It would make perfect sense if performance dropped in the new build because of settings higher than Default, but the performance hit happens even on the stock Default preset itself.
I see! 