Returning to the original question:

If you mean everyone boarding through the back door rather than the front door; then yes. Been quite awhile but I've done it a few times. (Explanation given that the jetbridge was not operational). Makes more sense than cancelling, or badly delaying a flight to change gates, even if it means some passengers with ambulatory issues may have to be rebooked. Have also deboarded out the back for the same reason (and once on some kind of test using both doors).

If you mean everyone boarded through the front door BUT starting with those seated in the back and working forward; then the answer is still yes. Twenty-five years ago, during my heavy business travel days, there were only two classes of seats. First loaded. Then coach by group starting from the back forward. AA did it all the time.

But Marketing and Revenue Management got involved. Hold baggage was no longer free, and seat classes started from lower steerage, then coach, super coach, business, and Flagship First. Overlaying it all is AAdvantage status from gold to finally the exalted private tiers that cannot be spoken about. So boarding had to change to something more representative of revenue and frequency. Which makes sense but costs efficiency. Even in the last year or so they "again" beta-tested fast-loading schemes; which all involve some version of sending people to the back sooner so the aisles don't get clogged.

But it is an idea doomed to rejection. Elite boarding groups are earned by frequency or paid for with money, and one of the features promised is boarding order (and implied with it overhead space).