There has been various ones over the years. These include e.g. key2Audio, Cactus Data Shield & Copy Control. They generally played without issues on normal CD p

There has been various ones over the years. These include e.g. key2Audio, Cactus Data Shield & Copy Control. They generally played without issues on normal CD players, but I did hear that especially car stereos did sometimes have issues with them. The way most of them worked were by attempting to hide the audio tracks from computer CD drives to make the ripping harder. I haven't really used audio CDs for ~15 years so I don't know what the situation is these days. At least the technologies I mentioned are no longer being used to according to Wikipedia. One way to tell if the disc has these kind of copy protections or not is to check if it has the Compact Disc Digital Audio logo. It's trademarked and Philips does not allow using the logo for CDs that break the specification. They do however allow setting the "no copy" bit (https://en.wikipedia.org/wiki/Bozo_bit https://en.wikipedia.org/wiki/Bozo_bit). No clue if that would be strong copy protection or not in Finnish courts.