Is your feature request related to a problem? Please describe.
Boss mechanics tend to change based on difficulty and this feature would make it easier to create raid-difficulty based conditions.

Describe the solution you'd like
Simply a global conditon that allows you to check for normal, heroic, mythic raid difficulty.
https://wow.gamepedia.com/DifficultyID

Describe alternatives you've considered
run custom code & add instance check there but that also requires all properties to be coded

Additional context
https://wow.gamepedia.com/API_GetInstanceInfo (3rd arg)
https://wow.gamepedia.com/DifficultyID
14 = normal
15 = heroic
16 = mythic

Can you list a few examples of actual abilities where that would be useful?

This sounds far too niche for my taste.

On shadowlands beta there is a boss (Huntsman Altimor) that applies a debuff (Sinseeker/Huntsman's Mark) that does different things based on which add is active. (mythic only)

Trigger1: Tracks debuff
Trigger2: Tracks unit characteristics - boss

condition:
if t1 and t2 npc id == xyz then do things.

this condition is repeated 3x

but i definitely used this more often in the past

You know, that you can now easily do that with a custom condition check?

Yea i know that there are custom coding solutions. I assume the one presented with "Run custom code" is better in this case