What would stop some major company from blacklisting specific Bitcoins based on the "inputs" used and their "origins"?

Question

I remember news about some exchange actually banning users based on some kind of "reputation score" on the inputs/coin parts they controlled/used, or got Bitcoin sent to them from. So even years and years back, they had this mapped out somehow.

Why can't they do the same today? Aren't they? If not, why? Why do they not take this obvious path to harassing and restricting people to only use Bitcoin in "approved" ways with "approved" people/entities?

Did they stop doing it for some reason? What stops them from doing this and thus locking out tons of people who have "dealt with undesirables"?

Answer 1

Because a transactions with 5 inputs and 5 outputs is possible and it doesn't make sense to ask the question "the output to me comes from which input?"

Simply, transactions are about total input value and total output value and the rest is miner fees. Tracking a coin's origin is not technically possible.