Released with Android 9 Pie in 2018, the built-in Markup utility on Pixel phones lets you edit (crop, add text, draw, and highlight) screenshots. The vulnerability was that screenshots cropped and sketched with it could be retroactively restored.
For example, if a person published screenshots from Google Pixel on the Internet, having previously cropped or painted confidential information, then due to the vulnerability, approximately 80% of the image could be restored.
Fortunately, most social media services reprocess uploaded images, which removes the associated data and reduces the impact of the vulnerability. For example, Twitter is protected from aCropalypse. However, screenshots uploaded to Discord before mid-January 2023 are already known to be vulnerable.
There is a test tool where you can upload a screenshot and see if the vulnerability affects an image you previously shared.