This would be incredibly useful, not only for runes but also talent abilities. For example:
#showtooltip
/cast [known:Mortal Strike] Mortal Strike; [known:Bloodthirst] Bloodthirst; [known:Shield Slam] Shield Slam
All of these abilities are mutually exclusive depending on the spec, and with SoD encouraging respecing this becomes really useful so you don’t have to constantly fix you action bar when respecing.
On the topic of runes you could also make something like this:
#showtooltip [known:Devastate,worn:Shield] Hands Rune Ability; [worn:Shield] Sunder Armor; Heroic Strike
/cast [worn:Shield] Sunder Armor; Heroic Strike
This will make the macro show the Devastate icon instead of the Sunder Armor icon when all of the criteria for Devastate is meet (Have the rune and a shield equipped). And if the player doesn’t even have a shield equipped then it will show and cast Heroic Strike instead.
Another rune macro that I would find useful is:
#showtooltip
/cast [known:Quick Strike] Hands Rune Ability; Hamstring
This will make the macro a rage dump (windfury proc) ability and cast Quick Strike if the Quick Strike rune is equipped and fall back to Hamstring if the Quick Strike rune is not equipped.
And my favorite, this would probably be the most useful warrior macro in SoD:
#showtooltip Charge
/cancelaura [noknown:Warbringer] Bloodrage
/stopmacro [noknown:Warbringer,combat]
/cast [noknown:Warbringer] Battle Stance
/cast Charge
This will cancelaura Bloodrage (due to it forcing combat) and stance switch only when the Warbringer rune is not equipped. It’s using [noknown], which is just the inverse of [known], to check if the rune is not equipped.
I see no reason to not include this feature into SoD or Classic WoW, it’s a really good Quality of Life and doesn’t allow automation. You can’t change runes or spec in combat anyway. It’s really annoying having to change and fix your action bar when trying out or experimenting with new specs and rune combinations.
What I think is the reason why it’s not in SoD is because this feature came with Dragonflight and Classic WoW is using an older client from BfA. My guess is that backporting this feature didn’t cross their mind (they did backport the new settings menu from Dragonflight and a bunch of Lua API changes though).
This would be a great QoL upgrade for many classes I think!