All the illusory walls in Dark Souls 3 as of the vanilla game are in this video and if you are having struggles with finding them, watch this video or check the Fextra Life wiki for the locations and descriptions of the illusory walls.
Wall 1 - In Firelink Shrine, below the nest of Pickle Pee and Pumple Rum, where the rafters are, beyond this wall lies the Covetous Silver Serpent Ring.
Wall 2 - Watchdog of Farron Bonfire, head out towards Farron Keep, but before taking the ladder down, turn left, then left again, and on the corner of the building lies an illusory wall with a crystal lizard if you keep proceeding around the tower. Betond this wall lies the Dream Chasers ashes.
Wall 3 - Abyss Watchers Bonfire, head towards the Catacombs of Carthus Bonfire, until you come across a room where you encounter the first "Kukri Throwing" Hooded Skeleton. Just as you enter the room, turn right and the illusory wall is right there and beyond it contains the Carthus Pyromancy Tome
Wall 4 - Catacombs of Carthus Bonfire, Head down the stairs were the Skeleton Boulder is, then turn left, where the "Kukri Throwing" Hooded Skeleton is to the right is the illusory wall containing stairs to get the Grave Warden's Ashes.
Wall 5 - Demon Ruins Bonfire, Head down the stairs to the main hall with fire orbs, then turn left, then left again at the first opening, jump down the hole and kill the rats, then go forward then left, then left again and the wall on the farthest right is illusory, beyond it lies the Quelana Pyromancy Tome.
Wall 6 - Demon Ruins Bonfire, Head down the stairs, and then again down the stairs until you are in the hall with the floating fire orbs. Go left until you see an entrance with a pit to drop in, jump in and once your in, the wall to your right is illusory.
Wall 7 - Just after wall 6, the wall beyond that is illusory (The wall behind the chest).
Wall 8 - Demon Ruins Bonfire, head down the stairs to hall with the fire orbs, then turn right, and at the end of the hall lies an illusory wall, beyond it is the Black Knight Greatsword and on NG ++, the Flamestone Plate Ring +2
Wall 9 - Old Kings Antechamber Bonfire, the illusory wall is in this room.... try find it... you'll get an estus shard for effort
Wall 10 - Church of Yorshka Bonfire, head the way back you came until you meet a balcony with a crystal lizard on it, to the right is an illusory railing and beyond that lies Dorhy's Gnawing
Wall 11 - Church of Yorshka Bonfire, head the way back you came, but this time go up to the throne/courtyard area as if your heading towards Pontiff Sulyvahn. Once your infront of the throne, the wall to your right is illusory and behind it is the Magic Clutch Ring.
Wall 12 - Church of Yorshka Bonfire, proceed through the graveyard and through the dark barn area until you get the stairs, the furthest right alcove under the stairs has an illusory wall in it with a crystal lizard in it
Wall 13 - From the Pontiff Sulyvahn Bonfire, head towards through the courtyard and the building at the end of the courtyaard has an illusory wall to the left once you enter it, therein lies 2 Sulyvahn Beasts and the Aldrich Faithful Covenant, of course Fath Mcdonell also
Wall 14 - Anor Londo Bonfire, head down the rotating lift and smash your R1 on the Gwyn statue to get a ring that has bismal use in game, and a wedding alter.... ugh, yeeehhh, wedding...
Wall 15 - Oceirus the Consumed King Bonfire, Proceed to where you would get Way of the Dragon Gesture, and the wall behind the chest at the end of the room lies and illusory, beyond it lies the Untended Graves
Wall 16 - Untended Graves, head towards firelink, then the place where Irina would be sitting has illusory walls around it.
Wall 17 - Lothric Castle, its in the room with the first Winged knight you run into, up the stairs then right from the bonfire, go up the ladder and the wall is near a bunch of barrels
Wall 18 - Grand Archives, go to the last place the Crystal Sage warps to, but instead of hopping down, or climbing down the ladder, turn left and the wall is goddam obvious