(TLDR at the end)
Before I say anything, Death Knights deserved the nerfs they received. They were near literally being unkillable. All of the changes were great besides the 50% reduction to Death Strikes healing based on player damage. This makes Death Strike heal 12.2% of damage over the last 5 seconds even though the tooltip says 24%. This ability costs 35 runic power and causes you to sacrifice damage if used during your burst. Death strike heals for 6.89% of your health at the minimum. You would need to take 55% of your health in the last 5 seconds in order to heal past that minimum threshold. This heal is still only roughly 33k.
One of the biggest reasons why DKs were unkillable was because of Will of The Necropolis. Will of the Necropolis does not work like you think it does. It states, “Damage taken below 30% Health is reduced by 17.5%.” This is not correct. This ability absorbs 17.5% of damage while you are below 30% hp. While it does not seem like there is a difference, this absorption is boosted by their talent Gloom Ward which increases absorption effects by 15%. Will of the necropolis is also impacted by Dampening. In solo shuffle, Dampening starts at 10% and is ramped up by 25% every minute. Every defensive that DKs have will be reduced by that amount besides Ice Bound Fortitude which is only 30%.
Death Strikes healing is increased by all damage you take, which includes Absorption effects. Previously, if you took 100k damage from players within 5 seconds you would be healed for 30% of that amount. Now that number is 12.2%. All damage done to your shields (AMS / Spellwarden / Will of The Necropolis) will count towards your next Death Strike.
In the context of Will of The Necropolis pre-nerf (Death Strike healing was 30% and Will of The Necropolis being 35%), if you took 100k damage while below 30% hp (135k raw damage before the reduction) your 30% heal from death strike will be based off the 135k damage. This would heal DKs for 40,500 instead of the normal 30,000.
This is very toxic in Battle Grounds since you can be hit by many people at once while healing more than you should due to Will of The Necropolis absorption. This is why DKs were beating healers on the BG Scoreboards. (Along with every DK defensive besides IBF showing on the healing charts while dodges and real damage reductions do not.)
Will of the Necropolis for Frost and Unholy need to be real damage reductions, not an absorb so it is not increasing Death Strikes healing while reducing its effect because of Dampening.
Currently, Rune of Spellwarden heals overall more than Death Strike (while actively pressing it) while it is a passive talent that how no counterplay or interaction with the DK at all. Rune of Spellwarden is boosted by 15% (Gloom Ward) and 20% (Unholy Bond’s 20% increase to rune forges). This creates unhealthy gameplay against Magic damage and no help against physical damage.
Again, with the changes to Death Strike, you would need to take 55% of your health in 5 seconds to heal more than ~33k (the minimum healing), which will be reduced by Dampening.
TLDR: Remove Spellwarden and make Death Strike heal 24% of damage from players (up from 12.2%) (it was 30% in Shadowlands)