Column Collapse started with a busy Jira board: a swimlane per developer, and every lane's Done column growing through the sprint until the work still in progress was pushed off the side of the screen. An existing extension could collapse a column, but only the first one, and on Jira's new board it squeezed the rest into uneven widths.
So this one puts a small button on every column header. Click it and that column folds into a narrow strip in every swimlane at once, the other columns widen into the space, and the strip still shows that lane's count. The count comes from Jira's own column header rather than from counting cards, because Jira only draws the cards on screen: a Done column of 45 has a handful in the page at any moment.
It's a couple of hundred lines of plain JavaScript, with no build step and no server. It reads only column names and counts, remembers which columns you collapsed on each board in your browser, and sends nothing anywhere. The store screenshots come from a made-up board that copies Jira's page structure with the real extension running on it, so no real work items appear in them.