That hides it from everyone but HTML pros, and even as an HTML pro myself, I'd typically not think to check the META if they've already designed the date entire

That hides it from everyone but HTML pros, and even as an HTML pro myself, I'd typically not think to check the META if they've already designed the date entirely out of the visible text. (I'd instead check for 1st appearance in the Wayback Machine.) I suspect the best hybrid strategy, for those who feel dates complete a duty to the reader, is to put the date in small, low-contrast text near the bottom. Perhaps also, when minor edits occur, update one or more similarly-subtle 'updated' dates. That emphasizes the material is not obsolete/stale, but live reference material still maintained.