From the top of my head, the only buildings / non-consumable items that are no longer useful in the end-game are wooden items (poles, chests) that can be burned as fuel, and burner inserters and miners.
I suggest adding these buildings as possible raw materials in smelters, yielding 1 iron each just like iron ore. Optionally, the steel furnace can be recycled similarly to yield 1 steel.
This is a very easy change that imho fits into the factorio philosophy and the current design decision that wooden items can be burned, and solves the annoying fact that you end up with a bunch of items you can't really get rid of except by shooting a container that contains the items.
(edit: I figured out it's pretty easy to add this in a mod: https://mods.factorio.com/mods/vanatteveldt/recycling (my first mod!
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