Bitcoin Forum

As in, traced back to the same user. Assuming you only had the BTC addresses and no other information.

Probably. Depends of address reuse and change addresses... There are many websites that allow blockchain analysis, ex.: blockchain.info (https://blockchain.info/), Blockseer (https://www.blockseer.com/) and Wallet Explorer (https://www.walletexplorer.com/). You may be surprised with the amount of info you find regarding known addresses or addresses you use...

If you want any privacy in bitcoin, you should use a wallet that offers "Coin Control" features, such as Bitcoin Core, and control your inputs and your outputs. There's no other way. If you don't, multiple addresses on the same wallet will in fact be linked and maybe you don't want that.

Segwit and lightning network to enable confidential transaction, coinjoin etc is the only way to improve privacy.

 Given an address, we can look up whether it was involved in a transaction or not.  If it was involved in a transaction, then it likely links to a change address on the same wallet.  If you allow the wallet to control spending, then that change address will be used in a subsequent transaction which would link at least those two transactions to the same wallet.  Continuing in this pattern, eventually every transaction could potentially link back to your wallet.

Use mixers, if so worry about privacy.