What items stack in lol??

Assuming you mean "what items can I buy multiple of while getting the full effect from the item each time"

Pretty much none.

If it says UNIQUE - Active/passive, then it doesnt stack.

Trust me....

The only stackable item that people still do (even tho is not efficient is ). If we count starter item are the only stackable item.

or . attack speed gutted

all that dont share the same "unique" name.

https://leagueoflegends.fandom.com/wiki/Item

The information your looking for is covered under "Unique" effects.

Basically stats stack unless listed as unique.

There are some items that may conditionally be worth stacking, even if their effects don't stack.

Also, some items won't even stack with other items, if they have the same named ability.

Also, slows don't stack.

Doran's Blade and Doran's Ring stacks. Doran's Shield does not stack.