I thought I was crazy, clicking EVERYwhere to get the "hide update" to show (for a Mac OS 10.12.5 Public Beta I no longer wanted to try).
Discovered something weird: I finally accidentally got the Hide Update ctrl-click to appear, and it depends on WHERE in the white space you click. In the expanded ("show more") view, if you click on the text, no submenu. If you click on the white space next to the update title, no submenu. Click on the icon, the update title, no submenu. Click on the whitespace BETWEEN the title and the first text header/paragraph, no submenu. But: Click on the whitespace to the right of the first text (which in this case says "Installation Instructions"), I get the "Hide Update" submenu! Also works to the right of any text (under the "update" button), or whitespace within the description text (but not the text itself).
Someone at Apple needs to get their z-order right for clickable hotspots & text fields.