That's the GP's point though: it already is impure! If you rewrite as

    let acc = {...}
    footballPlayers.reduce(...,acc)

It should become clear: acc is an external variable that got mutated by the reduce.

Indeed. A pure function should not depend on and / or mutate external variables.

I'd do it like this (yay for one-ish-liners!):

  const playersByCountry = footballPlayers.reduce((acc, player) => {
    return {...acc, [player.country]: [...(acc[player.country] || []), player]};
  }, {});