If the complaint was filed on a timely basis, the plaintiff has an extended period of time to serve the complaint in the defendant. Usually personal service is required. CCP sec. 415.10. While substitute service is authorized, it doesn't appear its requirements were met. See CCP sec 415.20

You do not provide enough facts to answer what I see are two questions.

The first about the Statute of Limitations (SOL) and the second about proper service. I am not sure what you mean by the SOL was "done" on February 29. If that was the last day, then the "claim" (assuming you mean a Complaint filed in Superior Court) was likely timely. If "done" means that was the first day after the SOL expired, then a Complaint filed in Superior Court is untimely unless there is an argument the Plaintiff / Complainant can make about tolling.

As for proper service, if the documents were simply left on your doorstep without any notice, then service is likely improper. Hopefully, you have a security camera to confirm that. However, if you answered the door and learned who was there and what their purpose was and refused to open the door once they confirmed your identify, a strong argument can be made that you were served because, after identifying yourself, you refused to answer the door and the papers were placed in front of your door. It's hard to determine based on what you've written, but assuming the "claim" (Complaint?) was timely filed and in light of the current COVID-19 / Coronavirus outbreak and the closure of the San Diego Courts, it is likely that your claim of improper service will hold up. I'm sure you will hear from the opposing party / their attorney when things return to "normal." All the best.

The statute of limitations is protected by filing the lawsuit prior to expiration. Service is a separate matter. Sounds like they filed timely.

As for service, no, leaving papers in front of your door probably isn't good service. An exception might be if you were talking to the process server through the door and refused to open the door to get served. Them leaving the papers might be good service. The person being served doesn't have to accept the papers to be served. Trying to refuse the papers does not defeat service.

If you were not home and papers were just left at the house would not be good service.

Please note that fighting over service of process usually isn't worth doing. You can spend a lot of time and money filing a motion to quash service, and the other side can fix the problem by just re-serving you.

You should consult with a local attorney about the case since you seem to know what it's about, and you can see how best to handle it.