Commands used in the video:
(cmd block/chat):
/scoreboard objectives add death-counter dummy "§f-§l§cDEATHS!§r§f-"
/scoreboard objectives setdisplay sidebar death-counter
/gamerule commandblockoutput false
(repeating cmd block):
/tag @a add dead
(chain cmd block/always active):
/tag @e[type=player] remove dead
/scoreboard players add @a[tag=dead,tag=!counted] death-counter 1
/tag @a[tag=dead] add counted
/tag @a[tag=!dead] remove counted
