We would like to be able to still have the expiration date value stored in the s2member_auto_eot_time, even after a member expires. It seems that the code clears or deletes this value.
Is there a way to prevent this from happening, to preserve the value in that field?
I am not aware of a setting for s2member to retain the EOT.
If you only want to retain this information for paying clients, this information is in your gateway records.
An alternative is to export s2member client records on a schedule. Make sure you have a robust security model though to make sure these data dumps don’t get leaked. See https://s2member.com/kb-article/advanced-importexport-tools/