As shown in screenshots in #366, the latest version of Identity mod (2.4.0) does not apply the Blindness effect to players whose current Identity is a Warden.

Wardens are supposed to be blind creatures; therefore, anyone disguised as one should get the Blindness effect.

scoreboard objectives add disguised_as_warden dummy

execute as @a store result score @s disguised_as_warden run identity test @s warden
effect give @a[scores={disguised_as_warden=1}] blindness 1

I don't think that is a good idea because it will ruin the player's experience.

If not Blindness, how about the Darkness effect? It still reduces effective view distance (to about 16-24 blocks max), but not as much compared to Blindness (which limits view distance to ≈3-4 blocks)

Or it gives the entities around them the darkness effect to make it so its like an actual warden.

Make this configurable in the ".minecraft/config/indentity.json5" file, I think in my opinion, it is a great idea for a more challenging experience to have a blindness effect as a Warden anyway, since this mob is already overpowered (also needs a laser ability), think about Wildmutt in Ben10 (yes, my childhood show, not a big fan of it now).

Yeah I think this is a good idea.

Done & done - you can check the wardenIsBlinded and wardenBlindsNearby to toggle darkness for yourself or other players around you. Have fun!