Bitcoin's decentralization is not controlled by the government and central banks, and it can be used to conduct free transactions without the government's knowledge and taxation.
Decentralization in essence means that a central authority or a system doesn't have the power to control how something works or what direction it will take in the future. Bitcoin transactions, like the majority of other crypto transactions, are not free. You don't pay a centralized entity, but you need to pay the miner to include your transaction in their block.
Depending on what services you use, your government could have the mechanisms to keep track of your crypto activities. If you use centralized exchanges where you have performed KYC, they could be cooperating with your government and share information about the citizens who transact on their platforms. Or a government agency could contact the exchange and request them to hand over information about a certain individual who they suspect might be laundering money or avoiding to pay their taxes.