But at the same time, the only potential harm in people not filling in descriptions is a screenreader user wouldn't be able to tell what the icon is. The potential harm if we offer a "is this animated?" tickbox and somebody doesn't tick it is, someone who was relying on that to block animations because they're sensitive to animations will then be exposed to an animated image that they were expecting us to block for them, and they've now been harmed by it.

I would much, much rather say "we can't guarantee that you will never be exposed to animated icons, so here is the information on how to block animations on the level of your browser" than say "tick this box to block animated icons. Maybe! If you're lucky and if people don't forget to identify their icons as animated!" The risk of causing harm is just too high.