1.16.4 - How to show the mob's health above his head?
Edit
/scoreboard could help.
scoreboard objectives add health health
# Put the following in a repeating command block
execute as @e[tag=track_health] run data merge @s {CustomName:'{"score":{"name":"@s","objective":"health"}}'}
Old answers
data merge entity @e[tag=track_health,limit=1] {CustomName:'{"nbt":"Health","entity":"@e[tag=track_health,limit=1]"}'
setblock ~ ~ ~ minecraft:sign{Text1:'{"nbt":"Health","entity":"@e[tag=track_health,limit=1]"}'}