At a previous job, we had an internal Wiki with a page titled: "Cow in a ditch" which was specifically for this purpose. Otherwise, it's mostly Confluence now,

At a previous job, we had an internal Wiki with a page titled: "Cow in a ditch" which was specifically for this purpose. Otherwise, it's mostly Confluence now, but no specific page of lessons learned, instead, those lessons are dotted around in individual documentation pages.