How do I find the serial number of an iphone i no longer own.

See this Apple support article—>Find the serial number or IMEI on your iPhone, iPad, or iPod touch - Apple Support

If you go the the website for the class action suit it can look up your serial number. If you are eligible you should have gotten an email with a link to the registration site. When you enter your Apple ID it can use that to look up the information about your device. If you didn’t get an email sent to your Apple ID email address you are probably not eligible.

This only works for current devices, not older devices you no longer have. I need my serial number from an old iPhone due to litigation against Apple for slowing down the device due to battery life.