FWIW in our rural county the code requirements for septic systems have changed a number of times. The bottom line is that a septic system that was installed earlier than just a few years ago will be judged noncompliant. If not rebuilt to the new code prior to a sale, the buyer will have two years to remedy. It has nothing to do with whether the system is "working."
This is what happened on one house we owned. County passed a rule that ANY sale of a house with a septic system, the septic had to be brought up to current code regardless if it was working or not. There was no physical way our septic could be brought up to current code as the lot wasn't big enough.
So we had to pay to reach and attach to city sewer... fortunately it was "only" a couple of blocks away.