In commander, there is the extra rule:
903.14a A player that’s been dealt 21 or more combat damage by the same commander over the course of the game loses the game
I am wondering as to the origins and intent of this rule. Why is commander damage tracked separately per commander and why is 21 the particular number chosen to be lethal?
Motivation for the question:
This rule is incredibly cumbersome from a game tracking standpoint. Even with just 4 players, it means that in addition to 4 life totals, there are 12 commander damage totals to track as well (or possibly even 16 commander damage totals if control changing effects are prevalent). This takes the number of things that need tracking well out of range of most magic tracking apps and counter methods, making pen and paper or large dice piles the only good tracking options.
My playgroup finds this so cumbersome that we ignore the commander damage rule almost entirely (we consider 20 damage from a commander in a single hit as lethal, and otherwise don't care). This decision was made under the key part of commander philosophy:
House rules or "fair play" exceptions are always encouraged if they result in more fun for the local community.
But this piece of philosophy also suggests that the commander damage rule originally was to create fair or at least interesting play.
Also, the number 21 is very odd in the context of magic. Magic game constants, aside from starting hand size, are usually multiples of 10: 20 life in a normal game, 30 life in two headed giant, 40 life in commander, 60 card minimum deck size in a normal game, 40 card minimum deck size in limited, 100 card deck size in commander...
[I'm posting my speculations as an answer, but I would love an answer with authoritative or historical sources.]