How much has changed about DKs since legion? Which spec are you referring to?
I can tell you how much Shadow has changed from Legion and it isn’t a lot actually. It is just a weaker version of what it was. The azerite traits and essences it needs in BfA are basically the artifact traits it lost albeit not as good.
At its core shadow has always been pretty much the same core rotation. Mind Blast on cool down, keep your dots up, and mind flay as filler. Void form added another element to it but it was never all that complex and still isn’t now. VB on cd while weaving in MB on cd, keep your dots up, and mind flay as filler.
That’s it. The only difference with void form is its ramping haste which speeds up the rotation and there in is the challenge - making sure to go through the rotation and maintain void form as long as possible. In Legion you used Void Torrent, Power infusion, and mindbender/shadowfiend at the right moments to extend void form for as long as possible so that mass hysteria could ramp up to some pretty crazy numbers. Surrender to madness only was viable really for the first tier. By the time Nighthold came around it was really not viable at all anymore.
In Legion it was incredibly punishing to a player who wasn’t using their abilities at the right time, or could not keep up with the pace of the spec. Heavy sustained movement also tanked the spec.
BfA attempted to remove that punishment by making void form cycles last less time, and by removing mass hysteria, but the core rotation from then to now remains completely the same just less options to extend void form.
The spec has traditionally performed bad in dungeon content that was past progression because it is a dot spec with few AoE options and what options it does have are relatively weak. Mobs simply wouldn’t live long enough to utilize shadow well in that content.
If you wanted to get into the minutiae of it all I would be more than happy to obliged as the priest class is my favorite class. One I have played since the very beginning of my relationship with the game. So much so that I have even bothered to play it at all in BfA and get it to my humble 451ilvl.
If you have specifics you, or anyone else in the thread, wants to discuss specifically then ask away. Let’s talk this shizz out. I want shadow to be fun in Shadowlands.