There does indeed seem to be no way of reading whether a certain achievement is gained. There is an event for it, but this wouldn't count achievements gained before your mod got added to the save https://lua-api.factorio.com/latest/events.html#on_achievement_gained

You could make an api request ...

I'm working on my first mod and I want to be able to read the status of achievements, specifically to know which ones are pinned by the user. Is this currently supported? I've checked the API docs and haven't been able to find anything, but I thought I'd ask here.