WowAce

add please the "out of sight" indicator when the target is in range, but standing behind a wall or something like that (an example of another addon https://imgur.com/sfWMxDm )

As far as i know is not possible to detect out of sight without casting some spell  (at least in official servers). Can you post which addon can do that ? And in which game version ?

Aptechka wotlk classic

the gif above shows an indicator that starts flashing red when I try to use healing on a target behind the wall

Yes but this only works when you start casting a heal, and the default UI already displays a red text saying:  "Target not in line of sight".

I dont know why this indicator is necessary, if the player is in range and the spell fails you already know that probably is due to the player not in line of sight (the other option maybe not enough mana).

in the turmoil of a boss fight or on BG/Arena, you may not immediately react that the target is out of sight, especially if you are trying to hang hots/shields