Dead days assistance:
Lower back, hams, abs
Bench days assistance:
Chest, triceps, lats, upper back
Squat day assistance:
Lower back, quads, abs, hams
That's how I've treated things since May.
Pick your assistance work as you walk into the gym. No need for pre planning that stuff. There are different types of accessory though. You have movements that are compound lifts that build a particular technical lift by mimicking at least a portion of the technical lift. These are your "main lifts". For example BB rows or floor press are main lifts that build your bench. Then there are supplemental lifts you would do after those. This would be skull crushers or band pushdowns.
My general rule of thumb:
Main lifts - singles to 10 reps. Vary it week to week. Usually starting at 10 reps next week 7 reps next week 5 reps. I typically switch these out every 3 week wave and I don't care for heavy singles on mains. Just my preference though.
Supplemental lifts - 10 to 100 rep range. Yes 100. Start doing 100 rep sets of ham curls or band pushdowns. These lifts isolate the muscle. Control the tempo and use a full ROM.
So a bench day example might be:
Flat bench: 6 sets of 3 at 80%
Floor Press: 4 sets of 5 at RPE 7
Incline dumbbell press 4 sets of 10 at RPE 8
Skull crushers 3 sets of 15 or to failure
Band pushdowns 1 set of 100
Heavy abs for 3 sets of 12