impressions fine, outbound clicks down 70% since mid june — keyword problem or the algo

Not a keyword problem. A lot of people reported that exact shape in that exact window — impressions holding or rising while outbound clicks collapsed. When a whole cohort moves together on the same date it's distribution, not your copy.

Mine partially came back by late August, call it 60% of where it was. I changed nothing at all. Everyone who happened to change something during the same weeks is now completely convinced their change worked, which is how half the advice in this space gets made.

One thing genuinely worth checking: whether the impressions are coming from the same surfaces as before. If home feed impressions quietly replaced search impressions you'd get exactly this pattern, because browse traffic doesn't click out and search traffic does. That's the one version of this where it's partly a keyword story — you'd be getting served to people who weren't looking for anything.

"fix your keywords" is what people say when they don't know. you're right that it should have moved impressions first.