How to make a script recognize when all players have died?

You would probably need a loop to set all values to true when the game starts:

local p = game.Players:GetPlayers()
for i = 1, #p do
    p[i].Alive.Value = true
    print(player.Name .. " is alive.")
end

And while the game is playing:

game:GetService('Players').PlayerAdded:connect(function(player)

local isAlive = Instance.new("BoolValue")
isAlive.Name = "Alive"
isAlive.Value = false
isAlive.Parent = player

    player.CharacterAdded:connect(function(character)
        character:WaitForChild("Humanoid").Died:connect(function()
            print(player.Name .. " has died!")
            player.Alive.Value = false

            local numPlrAlive = 0
            local p = game.Players:GetPlayers()

                for i = 1, #p do
                    if p[i].Alive.Value then
                        print(player.Name .. " is still alive!")
                        numPlrAlive = numPlrAlive + 1
                    end
                end

            if numPlrAlive > 0 then
                print("Players are still alive")
            else
                print("Everyone is dead!")
            end

        end)
    end)
end)