It's hidden because it's an advanced feature and bad coin selection can actually hurt you in the long run on fees and with some other traps (e.g. making dusty outputs, etc.)
If you need privacy where some UTXOs cannot be spent with others, make a separate wallet.
Yeah I guess you could just use a separate wallet for non-KYC coins and have another wallet for KYC'd coins. But back then Bitcoin Core did not have multi wallet support. I have no updated yet (im still using an old version) but I think the new Bitcoin Core versions have a way to quickly load wallets. In the older versions it always saved wallets as wallet.dat. Can you create a new wallet without having to delete the wallet.dat file? I guess it also asks you for a name for the wallet?
Also in the older versions you needed to change the name of your wallet.dat to wallet2.dat or something, and close the program and open it again so it again and have the wallet you wanted to load named wallet.dat, it was really annoying hence why I just have one single wallet for all transactions, which is now a problem if I want to get it audited to buy real estate or whatever since it's all within the same wallet. I guess I could have audit for specific addresses but I haven't looked into how this works yet.