Quickleaf
Legend
Indeed, that was a deliberate choice; with 5e players I’ve gamed with “leave a man behind” never seems to be something they consider acceptable - those situations always become fights IME.The obvious thing missing from those rules is any (very realistic) result where some of the PCs escape while others do not; i.e. the party has been split up with some of them still in combat or captured or killed or whatever.
OSR is another story. Then you’ve just got to run faster from the owlbear than your slowest party member

So there’s 2 ways to add that with these rules. 1) players opting to stay and fight (or, for example, petrified PCs) simply don’t roll for the Retreat, and they are not involved in the retreat at all. Easy peasy. 2) include a “the party is separated” bullet point on the list of consequences.