Actually, that fact is what defines fungibility. If some people don't want your currency while they're supposed to accept it, then it faces fungibility issues. When a government forces the exchanges to trace transactions that are connected with illicit activity and to deny the acceptance of those bitcoins, then they've essentially lost value, because compared with the rest, they aren't demanded from the X exchange.
My argument is that what a certain centralized entities do, does not define the attributes of that object.
Let me give you another example. Where I live we had a big inflation about 2.5 years ago and during that time many people started exchanging their cash for USD, some time later they sold it. One problem some people faced was that some exchanges didn't accept dollar bills that had an old date.
So with OP's logic we should claim that US dollar (cash) is not fungible which makes no sense! Not to mention that they could dump their USD elsewhere that didn't have that dumb rule.
If you go to a grocery store you can pay with any "coins" you have in your wallet regardless of its "imaginary taint".
If I lend you 1
BTC you don't have to pay me the same coins, you can use a different set of UTXOs to pay the 1
BTC back.
If you have 1
BTC it can be split into the same smaller denomination as any other 1
BTCThat means BTC is fungible.
Now if you go to a centralized exchange and deposit some coins that exchange can suddenly decide that it doesn't want to give any services to your country and block your account and money (see it had nothing to do with "taint", etc.) but that doesn't change anything about fungibility of bitcoin.