It depends on how the owner treats it. I was really pondering on this and decided to share to know what others feel about this. If Bitcoin is preferably kept in Centralised exchange i think it is no longer decentralised to the owner because the don't have the keys to unlock and spend their coin but rather the rely on this third party intermediaries who decide how much the are eligible to transfer depending on their KYC verification level.
I believe private keys is what makes every Bitcoin owner decentralised not just the coin itself so if CEX is preferred over DEX and hardware wallets specifically open sourced wallets then the idea of centralised slavery is what should be dealt with. The fear of misplacing seed phrase and private keys is still a big concern to many who believe they are careless with storing and keeping important datas.
In my opinion if decentralisation were to be a coin then no matter where it is stored it should be accessible no matter what happens, even if the exchange fails. But because it is dependent on where the coin is stored then it is what the coin owner makes of it. It is decentralised for non-custodial wallets holders as the can always prove the coins are theirs anytime and centralised for custodial wallet holders because the can only access their funds if the are well verified and provided the exchange hasn't collapsed.
