If you are the one using a programming library directly, you may know that `prefer` does this. If you are using a client that doesn't expose this to you, you may not know that you are using an unencrypted connection after you had set ssl on.
No, that's a bad take. We should encrypt everything. The reasoning for not doing so in the past was computing power. Nowadays, that's really not a problem. Even my personal servers in my home network are all operating with encrypted links - because why the heck not? At work, everything that can be encrypted is encrypted, otherwise it's a bug. Even if currently 'there's no way' for attackers to get to that particular network. Emphasis on "currently". We shouldn't be a compromise away from leaking everything. Defense in depth.
Plus, the more you are familiar with setting up encryption, the less tempted you are going to be to cut corners when you are setting up something that's at a higher risk.
All the SQL transit is cleartext by default.
A "wallet" can be defined for TLS, but the Oracle Instant Client doesn't (appear to) support this functionality.
I push all my local Oracle database links over stunnel.
Oracle SQL Developer even has an SSH pane, because wallets are so very unusable.