Using effects Instant healing and Instant damage doesn't affect any mob at all, when using /effect command, they don't take take damage or receive health according to type of the mob or effect, it just does nothing. It makes some command block creations broken, and it's a technical issue that should be fixed to make those creations working again.
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I can partially confirm this (in 1.16.20 on Windows 10).
Undead mobs are not affected by /effect commands used to apply Instant Damage or Instant Health affects to them.
However, regular mobs are affected by both effects.
The mobs I tested included creeper, skeleton, zombie, witch, spider, stray, husk, and drowned. All commands displayed particles and wrote the message "Gave effect * 1 to mobtype for 1 seconds". However, neither the Instant Damage effect nor the Instant Health effect appeared to change the health level of any undead mobs. I tested each of these effects for causing either damage or health to the mob. Repeated applications did not cause the mob to flash red and eventually die, nor did it change the number of sword strikes it took to kill the mob.
Steps to reproduce:
Create a flat world with Default Game Mode set to Creative, Difficulty set to Normal and Mob Spawning disabled. Open it.
Obtain an unenchanted diamond sword.
Build a glass box with a 1 x 3 x 1 interior void.
Knock out one block at face level (so you can hit mobs with a sword).
Summon an undead mob (zombie, skeleton, stray, husk, or drowned) inside the glass box.
Issue the command /effect @e[type=!player] instant_health.
Expected result: The mob flashes red, indicating it took damage. If repeated enough times, the mob dies.
Actual result: The mob appears unaffected and does not die no matter how many times you use the command.
Hit the mob twice with the sword.
Issue the command /effect @e[type=!player] instant_damage.
Hit the mob once more with the sword.
Expected result: The mob survives because the instant_damage partially healed it.
Actual result: The mob dies. These mobs normally die after 3 hits with an unenchanted sword, but the Instant Damage effect should have healed it enough to survive the 3rd blow.