So I made this farm and not a single slime has spawned in the several hours that I was AFKing. Now I admit that I haven't lit up all the caves around the farm, but at least 70% are lit up, even then it shouldn't make that big of a difference, yet no slimes are spawning at all. I'm not expecting amazing rates, but at least a couple of slimes should have spawned while I was AFKing.
I didn't have any string to place around the top platform, but it's said that it will only improve the rates, they're not necessary for the spawns.
I did build an item collection system / elevator to bring the slimeballs to me on the surface, and it works I checked it with multiple items.
Several slimes spawned on the bottom platform while I was digging, so I didn't miss the slime chunk.
Firstly, all those torches are completely unnecessary. Mobs cannot spawn inside solid blocks, so all you need to light up are the cave holes and the surface (and, of course, the platforms you want slimes to spawn on). There does not seem to be anything visually wrong with your farm, so the obvious issue is that you haven't spawnproofed nearly as much as you think you did.
Because most people just hop into caves or dig out mines in order to spawnproof stuff, it's extremely easy to miss dark spots that you should be lighting up. Cave frequency/density is not uniform across any particular area of the world, there can be places where there are fewer caves and there can be places where there are more caves. You aren't really going to know unless mobs spawn in them or you resort to cheats (ie, spectator mode). A much better way is to dig regular vertical holes across the area you wish to spawnproof; it's probably easiest to keep track of if you dig one hole in each chunk, but given the average sizes of cave pieces you can probably get away with holes every 3rd chunk or so.
yeah i checked the video it was posted on the 2nd November 2019 the farms ideal functions which it used to get the slimes has probably been changed in one of the updates which caused the farm to break,